A crowd of more than 200,000 packed Belgrade's Republic Square on Wednesday night to celebrate his victory, and when he climbed onto the stage to speak they erupted into a massive roar. Dr Vojislav Kostunica, lawyer, dissident and opposition leader, was crowned that night by the men and women on the streets of Belgrade and 30 cities across Serbia as the new Yugoslav President.

"They tried to steal our election. They tried to negotiate about our votes. There is no second round. There is no dealing with votes and democracy. The result is that we have won and they have lost," he declared. Cheers rose from the square and flags waved furiously.

But Slobodan Milosevic remains in power and is contesting the outright victory Kostunica claims.

Who is this challenger who has emerged from being a total unknown in the West to become one of the most frequent names to crop up in newspapers and on television over recent days?

Kostunica has been on the opposition scene in Yugoslavia since the early 1990s. The 56-year-old is a modest, quiet man. He drives a Yugo car, like many of his countrymen and has a Belgrade flat he shares with his wife - also an academic - and two cats. They have no children. One of the features that makes him attractive after more than a decade under Milosevic is that he is very ordinary.

He's not exactly what you would call dynamic. At the end of one press conference, he needs to pass a journalist who is hurriedly stuffing notebook and pen into her bag. Instead of brushing past, he stands and waits for her to finish. Kostunica does not have the arrogance of power.

In a vigorous election campaign in which he has criss-crossed Serbia in a modest few vehicles, he has met people from across the country.

But unlike Western campaigns, he has gone to meet people in their communities, not to be seen on television out on the campaign trail - for in Yugoslavia, Milosevic controls national television.

As part of that campaign, he trailed from the Bosnian border town of Loznica to the Bulgarian border town of Zajecar; from Nis in the south to Novi Sad in the north. He also made a journey into the Serb part of Kosovo.

Milosevic dare not go to Kosovo, whose divided Serb population is angry. But Kostunica walked into the lion's den. Yet even he was not prepared for the apples, tomatoes, eggs and potatoes that were thrown at him. "I am ashamed that this has happened, because I am a Serb," he said. "But I am pleased because it means that Milosevic is desperate."

By the end of the day, the pro-Milosevic Serbs of Kosovska Mitrovica had smashed the windows of his three official cars and kicked in the bodywork, and he was lucky to return to Serbia alive. But he never lost his temper. He blamed Milosevic agitators. And he talked, as always, of a Yugoslavia under his presidency which would have no revenge.

A few days before the election a vitriolic article appeared in the Yugoslavia state newspaper Politika, the mouthpiece of the regime and a propaganda sledgehammer. It was the first of a two-part piece about Kostunica. The article - a massive character assassination - reflected the regime's fear of the opposition challenger.

Politika attacked him for living with cats (they said he lived with 17) and for having mistresses, which is par for the course in Serbia (only it said he was only interested in girls under 25).

His other main fault was that he lacked humour: "He just does not know how to laugh. And he is stiff as if he was wearing a bullet-proof vest. Narrow-minded. Hairsplitting. Stubborn. Stickler and boring. Tense somewhere inside. Cold. It seems that there is no life in him. No trace of emotions."

The problem about Kostunica is that not much mud sticks. He is not a man of strong, stark colours. He has the same greyness as John Major. He is quiet, modest, honest - a bit boring.

In the revolutions of Eastern Europe a series of charismatic leaders emerged: Lech Walesa, and Vaclav Havel, perhaps the best known. Kostunica is a different kind of man. He even suggests that he is just "a mechanism" by which Milosevic can be removed from power.

He admits that he hates election campaigning and perpetual interviews. But he realises that he has been entrusted with a job - and he will carry that job out. And Politika is right about part of his character.

He is low key, but he is a stickler for things to be done according to the rule book, for things to be done properly. He is stubborn. In the town of Loznica, people listened and said they believed he would stand up to Milosevic - he would have the courage that was needed.

In the last few days, as Milosevic has sought to buy time by pushing for a second round runoff in the presidential elections, some opposition thinkers apparently advocated going for a second poll.

Kostunica is an anorak about democracy. He studied law at university and then specialised in constitutional law. When the idea was raised that the sacred votes people cast for him should be traded in some political deal to help Milosevic, this quiet-spoken man reportedly yelled across the table that he would not do it. The name Kostunica means "tough, like a walnut" and the 56year-old lawyer and anti-Communist dissident will now have to use and show all his toughness.

Kostunica's family comes from central Serbia, the heart of Serbian nationalism. This - together with his carefully moderated nationalist rhetoric - has won him a place in the heart of many Serbs who are not drawn to other pro-West opposition leaders.

Is he a nationalist? Long-time colleague Dr Svetozar Stojanovic says Kostunica "is a man who is serious about democracy, but he is also serious about national interests".

Kostunica was born into an intellectual Belgrade family, the only son of a lawyer who was kicked off the Serbian Supreme Court for failing to comply with Communist justice. The young Kostunica soon began to follow the family record of dissidence. He studied law in Belgrade, then gained a masters degree and did a doctorate - a study of the political opposition in capitalist systems - that formed the basis for his first book.

He lost his university job after he gave support to political critics of Marshal Tito's new 1974 constitution. After he left academia, he gained renown as a legalist and human-rights activist. Then, in the same year the Berlin Wall fell and Communism across the East began to collapse, Kostunica jointly founded The Democratic Party - Yugoslavia's first alternative party to the Communists - and became an MP.

He later founded his own party, the Democratic Party of Serbia, and was chosen as opposition presidential challenger after research showed he was the leader for whom most people would vote.

Kostunica sees these elections as a defining moment: "It is a matter of life and death. These are not some sort of genuine elections. We are having a referendum on Milosevic."

That referendum was won on Sunday night, but Kostunica's battle for peaceful transfer of power was not over on election night.