Russia is holding an official day of mourning after 39 people were killed and more than 70 injured in twin suicide bombings on the Moscow Metro.
As Russian President Dmitry Medvedev vowed to "destroy" the perpetrators, security was stepped up amid fears of fresh attacks.
No group said it had carried out the bombings but officials blamed Muslim militants from the North Caucasus.
Police are said to be seeking three people sighted along with the bombers.
The United States vowed to help bring to justice the perpetrators.
Russian President Dmitry Medvedev said on Tuesday he did "not rule out" tightening anti-terrorist legislation.
Two suspected female suicide bombers detonated bombs packed with pieces of metal at two separate stations on the same line during rush hour on Monday morning.
But the line on the Metro, one of the traffic-clogged Russian capital's most vital commuter assets, had resumed traffic by early evening on Monday.
"It's really terrifying," Vasily Vlastinin, 16, told the Associated Press news agency.
"It's become dangerous to ride the Metro but I'll keep taking the Metro. You have to get to school, to college, to work somehow."
Russians have been lighting candles and laying flowers in memory of the victims of the blast inside the Lubyanka metro station, where at least 23 people died, and the Park Kultury station, where the second explosion killed at least 12 people.
Another four people died in hospital, and officials have warned that the death toll could rise.
The main television channels have changed their schedules, dropping advertising and entertainment programmes.
Prime Minister Vladimir Putin, visiting the injured at a hospital in Moscow, said law enforcement agencies would "do everything to find and punish the criminals".
Mr Medvedev laid a wreath at the scene of one of the attacks on Monday. He called the plotters "beasts", adding: "We will find and destroy them all."
Russian Foreign Minister Sergei Lavrov told the Interfax news agency that militants operating on the Afghan-Pakistan border may have helped organise the Moscow attacks.
"We all know very well that clandestine terrorists are very active on the border between Afghanistan and Pakistan," he was quoted as saying.
"We know that several attacks have been prepared there, to be carried out not only in Afghanistan, but also in other countries. Sometimes, these journeys go as far as the (Russian) Caucasus."
US President Barack Obama pledged that Washington would "help bring to justice those who undertook this attack" while Secretary of State Hillary Clinton called terrorism a "common enemy".
"Whether you are in a Moscow subway or a London subway or a train in Madrid or an office building in New York, we face the same enemy," Mrs Clinton said in an interview with the Canadian network CTV.
Other foreign ministers from the G8 group of leading industrial nations also condemned the attacks at the start of talks on global security in Ottawa, CanadaPolice are looking for two women who accompanied the bombers as well as a possible male accomplice, after identifying them and the bombers through surveillance footage, security sources were quoted as saying by Russian media.
The head of Russia's intelligence service, the Federal Security Service (FSB), said investigators believed the attacks had been carried out by "terrorist groups related to the North Caucasus".
"Fragments of the bodies of two female suicide bombers were found earlier at the scene of the incident and examinations show that these individuals came from the North Caucasus region," Alexander Bortnikov said.
The co-ordinated attacks were the deadliest in Moscow since February 2004, when 40 people were killed by a bomb on a packed metro train as it approached the Paveletskaya station.
Six months later, a suicide bomber blew herself up outside another station, killing 10 people. Both attacks were blamed on rebels from Chechnya.
Chechen rebel leader Doku Umarov claimed responsibility for that attack and vowed last month to take the war to Russian cities.
Chechnya's Kremlin-backed leader Ramzan Kadyrov condemned the attacks in Moscow, saying he would assist the Kremlin in hunting down the culprits.
More than 100,000 people have been killed in 15 years of conflict in Chechnya, and low-level insurgencies continue there and in the neighbouring republics of Ingushetia and Dagestan.
The city's Metro is one of the busiest underground railways in the world, carrying about 5.5 million passengers a day