But who planted them? There is more than one group that may have a motive as well as the means to carry out such attacks.

The Dagestan connection

Most Russians are making a connection with the events in Dagestan, where Islamist guerrillas are fighting Russian government forces.

Several thousand militants are currently fighting the Russian army in the southern republic of Dagestan.

Most entered Dagestan from the neighbouring republic of Chechnya, which has been beyond Russia's control for the past three years.

The mayor of Moscow, Yuri Luzhkov, has said he is confident the explosions are the work of forces originating from Chechnya, rather than Dagestan itself.

Rescue workers have been on duty for days

Over the past few weeks the militants have gained possible motives for revenge as Russian planes have bombed their villages in Dagestan and Chechnya and killed their comrades.

That makes a North Caucasian militant, either a freelancer or someone working directly for the militants, the most likely suspect at the moment.

However, none of the militant leaders has yet claimed responsibility. The leading Chechen warlord, Shamil Basayev, told a news agency that neither he nor his Saudi-born ally, Khattab, had anything to do with the explosions.

Muslims from the North Caucasus are often regarded by Muscovites as potential criminals and troublemakers.

President Boris Yeltsin has warned that the explosions could have been caused by forces hoping to destabilise the government by causing inter-ethnic tensions.

"One must not shift the problem onto an ethnic or religious basis. It is impermissible to carry out purges along ethnic lines," he said.

Other theories

Although most people still blame Islamic guerillas, there are other groups in Russia who have a history of violence.

Scores of survivors have been left homeless

Criminal gangs are increasingly well-armed and ruthless and have used car-bombs to blow up their enemies for a number of years.

It is possible that someone in each of the targetted buildings was the victim of a mafia assassination, though this is not thought to be a likely explanation.

Maverick political extremist groups have also been suspected of staging a series of acts of terror recently that include blowing up a statue of the last tsar Nicholas II and the still unsolved murder of the popular liberal politician Galina Starovoitova.

But many questions still remain unanswered: Why were bombs planted in ordinary Moscow apartment blocks? What did a terrorist hope to achieve apart from sowing panic in the Russian capital?