The area around the MI6 building in London was sealed off last night after being rocked by the two explosions. Eyewitnesses reported seeing smoke over the top of the building, home of the foreign intelligence service.
The ambulance service said nobody had been injured in the blasts which were reported just before 10 p.m.
Firefighters were called to the scene at Vauxhall Cross at 9.52 p.m. The cause of the explosions was not immediately clear.
Eyewitness Mr Andrew Preece was driving home with his girlfriend when he saw a flash of light coming from the top of the MI6 building.
He told BBC News 24: "It looked as of it was internal from the centre of the building but the flash of light was from the top of the building.
"It was followed by a large bang and the ground seemed to shake and my car was shaking. I looked at my girlfriend and said, I think that was a bomb.
"We drove off along the Embankment trying to look at the building trying to see something and I didn't. Coming back, all the roads were closed off."
A London Ambulance Service spokesman said they were called to the scene just after 9.50 p.m. after reports of an explosion and two crews were sent.
He said: "But we have had no reports of any casualties or injuries so I think an hour and a half later it is safe to assume there are none.
"We now only have one ambulance and one duty officer at the scene."
An eyewitness near the MI6 building said he heard two blasts and saw smoke pouring over the roof.
Mr Sridharan Balakrishnn (23), who works at the Texaco garage on the Albert Embankment behind the building, said: "I heard two large bangs at around 9.55 p.m. It was very loud and hurt my ears.
"Then I saw smoke coming from the MI6 building.
"I was very scared and I went into the office at the back of the garage and phoned my manager to ask him what to do.
"I didn't call the police because they were here so quickly - it took around five to 10 minutes.
"There were two vans and two car loads. Then an ambulance came.
"I didn't see anyone being carried out of the building."
Police sealed off a huge area around the building and closed the road between Vauxhall Bridge and Lambeth Bridge.
The building, headquarters of MI6, is a distinctive cream and green structure on the south bank of the Thames about a mile from the Houses of Parliament.
It featured in the last James Bond film, The World is Not Enough, when part of the building was shown being blown up in one scene by a bomb smuggled in by a terrorist.
Mr Matt Tiller, who lives near the MI6 building, told BBC News 24 the explosion was "absolutely massive".
Another eyewitness, Mr Alex Frank, said he heard two explosions from his bedroom across the railway from the building, which is less than one mile from parliament.
He said: "I heard two distinct explosions and massive white clouds over the railway tracks.
"There were definitely two explosions of the same strength. They were loud enough to shake the building I'm living in."
Mr James Trott (42), from Hull, east Yorkshire said: "I was coming over Vauxhall Bridge in a minibus and I heard two explosions coming from the MI6 building.
"We looked up and I saw panes of glass shattering on the pavement. "A couple of the windows had been blown out on the eighth floor.
"Police, ambulances and fire engines all arrived minutes later."
Ms Theresa Harding, who lives in the area, told PA News: "I was in my lounge and we were watching the Addams Family on TV.
"I heard an almighty bang and then another one seconds later. It sounded like an extremely loud firework. "Then I came to have a look and all the police were arriving in droves."
MI6 answers to the Foreign Office and last night a spokesman there said: "We are aware of the incident, but we don't know any details as yet.
"The police are investigating, but we are not aware of any casualties at the moment."
The spokesman added: "(Foreign Secretary) Robin Cook is being kept closely in touch with developments."
Liberal Democrat defence and foreign affairs spokesman Mr Menzies Campbell, speaking from the party's annual conference in Bournemouth, said: "This is clearly a serious breach of security.
"It's a relief that, so far as we know, there are no casualties.
"But obviously very substantial questions must now be raised about the protection of those who work in the security services."