An exhibition of the Renaissance artist Titian's work shows why he was feted by the nobles of Venice, writes Lara Marlowe in Paris

Tiziano Vecellio - whose name is anglicised as Titian - was the most famous painter of the Venetian Renaissance. Through much of the 16th century, he decorated churches and palaces, painting holy virgins and alluring Venuses with equanimity.

Titian apparently saw no contradiction between religious faith and an appreciation of the courtesans who were an integral part of life in the liberal city-state. His rich colours, particularly Venetian red, are instantly recognisable.

Titian bequeathed to the history of art the reclining nude and the equestrian portrait. Both are represented in the Musée du Luxembourg's fine exhibition, Titian: the Face of Power.

In it, Nicola Spinosa, superintendent of the Naples museums, has brought together paintings from 14 countries, including 35 Titians and 25 comparative works by Rubens, Lorenzo Lotto, Parmigianino and Tintoretto.

Titian painted kings, popes, intellectuals and beautiful women, all of them wielding different kinds of power, hence the title of the exhibition. As Giorgio Vasari wrote in his Lives of the Artists: "There has been hardly a single lord of great name, or prince or great lady who has not been portrayed by Titian, a painter of extraordinary talent in this branch of art."

Titian was born to a family of local nobles in the foothills of the Dolomite mountains, between 1488 and 1490. By the age of nine or 10 he showed such artistic talent that he was sent to Venice as an apprentice. He ended up in the workshop of Gentile and Giovanni Bellini, where he met Giorgione, whose arcadian style he adopted. Giorgione's death from the plague in 1510, and Giovanni Bellini's great age, brought Titian success in his 20s. When his master Bellini died in 1517, Titian became the official painter of the Venetian Republic, by offering to paint the Great Council Room of the Doges' Palace free of charge.

All his life, Titian was known for his business acumen. Like many a modern-day builder or house painter, he began adding on expenses once he'd landed the contract, demanding that the republic of Venice rebuild the roof of his studio.

Despite his high prices, he could not keep up with demand. In 1537, Venice briefly suspended his stipend because he was so far behind on work commissioned by the city.

Titian's most important encounter was with Charles V when he was crowned Emperor of the Holy Roman Empire in 1530. Having inherited Spain, Flanders, Latin America, parts of Italy and the Hapsburg possessions, Charles V was one of the most powerful men who ever lived.

"The invincible emperor was so pleased by Titian's work that, once they had met, he would never be painted by anyone else," Vasari wrote. "Every time Titian did his portrait he made him a gift of a thousand crowns in gold. His Majesty also ennobled Titian, giving him a pension of two hundred crowns . . ."

An anecdote recounted by the Italian art historian Carlo Ridolfi a century later shows how highly the emperor regarded the painter. One day when Titian was painting, he dropped a paintbrush, which the emperor picked up.

"Sire, your servant does not deserve such an honour," Titian said.

"Titian is worthy of being served by Caesar," the emperor replied.

An equestrian portrait of Charles V at Mühlberg, where the emperor conquered Protestant princes in 1547, was later imitated by Rubens, Van Dyck, Velázquez and Rembrandt.

Fourteen years earlier, Titian painted Charles V more simply, wearing the black clothing then in fashion with Venetian aristocracy. Without the trappings of power, the emperor looks like an ordinary human. Titian's 1533 portrait conveys a sense of disquiet, and hints at the mysticism that would lead Charles V to abdicate and retreat to a monastery two years before his death.

The emperor spent much of his life fending off threats from the French King Francis I, the Turkish Sultan Suleiman the Magnificent, Roman popes and Protestant reformers. In the exhibition, coats of armour and a one-eyed knight remind us that it was a century of religious war.

Titian infused his portraits with character, as if he could read the souls of his sitters. Though the faces are immensely expressive, they are difficult to interpret, as in real life.

One of Titian's duties as Venice's official artist was to paint the doge, the official elected by an assembly of patricians to rule the city-state. Doge Marcantonio Trevisan seems intent on communicating with us. The cloth bunched up in his right hand conveys a certain tension. But the look on the strong, middle-aged face might be one of wisdom, sadness or cruelty; we cannot be certain.

Titian portrayed women as thinking, feeling, sensual human beings. In most cases, their power is the power of seduction. But Isabella d'Este, Marquise of Mantua and one of the great patrons of the arts of the Italian Renaissance, also had the power of intellect and money. Titian met her when she was 60, and proposed painting two portraits: one as she was at the time, and a second, based on canvases executed in Isabella's youth.

The realistic portrait was lost, but we know from a copy by Rubens that it showed an overweight, over made-up Isabella with dyed hair.

By contrast, the portrait of the young Isabella, which Titian based on a 20-year-old painting by the Bolognese painter Francesco Francia, portrays a determined, attractive and eccentric young woman.

Titian never shrank from flattering his subjects - in portraits of the Hapsburgs, for example, he minimised the prominent family chin. But even Isabella had to admit he'd idealised her.

"The portrait by the hand of Titian pleases us so much that we doubt having been, at the age represented, of the beauty that appears here," she said.

The noble families whom Titian painted were called Este, Gonzaga, della Rovere and Farnese. Visiting the Musée du Luxembourg's exhibition is like walking through a pictorial family album.

Portraits of Isabella d'Este's son Federico II Gonzaga, and her brother Alfonso, Duke of Ferrara, Modena and Reggio, hang near her.

In the preceding room, devoted entirely to women, is a superb portrait of Laura Dianti, who became Alfonso's official mistress after his second wife, Lucrezia Borgia, died in 1502.

Most of Titian's models had the strawberry blond hair which the French call Venetian blond. But Laura Dianti's dark hair is swept up into a gold, pearl-encrusted turban. The young woman wears a royal blue gown with a gold shawl. The gold colour is repeated in threads on her sleeves and in the clothing of an African page who holds Laura's gloves. Ethiopian boy servants were a status symbol for 16th-century society ladies.

Titian also painted his friends, including Pietro Aretino, the boisterous, cynical, well-connected dramatist and poet who called himself the "secretary of Europe".

When Aretino moved to Venice after the sack of Rome in 1527, he and Titian became close friends, with Aretino acting as Titian's agent and artistic adviser.

In Titian's portrait of Aretino, the writer wears the gold chain which Francis I of France gave him to express gratitude for a Titian portrait. The luxurious Venetian-red robe emphasises Aretino's bulk. His long beard is neatly brushed. Again, the expression is ambiguous; it could be anger, amusement or rapt attention.

In one letter, Aretino called this portrait "a terrible marvel". In another, to Cosimo de Medici, he said: "It breathes; the pulse beats; the mind works as mine does in life."

But then, referring to Titian's legendary stinginess, he added: "If I had given him more ecus, the cloth would be truly shining, soft and full of texture, like satin, velvet and brocade . . ." To Titian, Aretino complained: "The portrait is more sketched than completed."

Even as an old man, Vasari wrote, Titian was "in sound health and as fortunate as any man of this kind has ever been; from heaven he has received only favours and blessings".

Perhaps the greatest blessing was to be remembered through five centuries as a Renaissance genius, while most of those he painted were forgotten.

"In fact, the one who holds real power is the painter who met all these people and painted their portraits," says Nicola Spinosa, the curator. "That is what we want to show with this exhibition."

Titian: the Face of Power is at the Musée du Luxembourg in Paris until Jan 21. Open every day. Tickets on www.billet-coupe-file.com. For more information see www.museeduluxembourg.fr