The police presence in Moscow is three times its usual size and people have been told to be on alert for signs of anything unusual.

Police are now holding more than 20 suspects on charges of involvement in terrorist acts, according to the Itar-Tass news agency.

Vehicles entering and leaving Moscow are being stopped, the police are making random document checks all over the city, and a widespread search is under way for explosives.

A policeman checks the cellar of a block of flats

In particular, police are looking for a consignment of nearly two tonnes of explosive believed to have been brought in from the Caucasus region concealed in bags of sugar.

Moscow Mayor Yuri Lushkov has told the police that every market, every shop, every warehouse, every entrance to a block of flats and every basement in the city has to be searched.

In the big blocks of flats - where most Russians live - people have been setting up their own watch committees, registering the names of all those living in their neighbourhoods and checking empty rooms.

BBC Moscow Correspondent Robert Parsons says it is almost as if the city is on a war-footing.

'A death for a death'

Moscow correspondent Robert Parsons: "Security services cagey over call"

A man with "a Caucasus accent" and claiming to represent an organisation called the "Dagestan Liberation Army", telephoned the Russian news agency Itar-Tass saying his organisation was responsible for the recent bombings, the agency reported.

	Chechnya: Round two? Battle for the Caucasus Cat-and-mouse conflict Islamic roots in Dagestan Russia's demoralised army

He described the three Moscow blasts and the bomb at a block of flats in Buynaksk as "retribution" for the Russian air force's "terrorist actions" in Dagestan and Chechnya.

"We wish to state that not one of the Russian pilots' bombs will go without a response. Not one of the lives of our children and women will go unavenged. There will be a death for a death.

Russia claims its army has made progress in Dagestan

Prime Minister Vladimir Putin has directly accused Chechen rebels fighting Russian troops in the southern republic of Dagestan of responsibility for the explosions.

The rebels have declared a holy war against Russia and are trying to create an independent Islamic state uniting Chechnya and Dagestan.

Mr Putin says he wants a security cordon thrown around Chechnya.

According to the Interfax news agency the Russian Defence Minister Marshal Igor Sergeyev said "several thousand guerrillas" are concentrated in Chechnya near the border with Dagestan.

He said that the army was "fully prepared" for further incursions into Dagestan from Chechnya.

Emergency laws

However, the prime minister has ruled out any possibility of imposing emergency measures inside Russia in response to the bomb attacks.

"It is time to stop talking about a state of emergency", Mr Putin told journalists on Tuesday.

Gennady Seleznyov, speaker of parliament's lower house or Duma and a Yeltsin critic, said on Wednesday that he too was opposed to a state of emergency.