Russia will not be cowed by terror attacks after a bomb on a commuter train killed 42 mainly young people, President Vladimir Putin has said.
The same rail service was bombed in September
Most of the dead and some 200 injured in the rush-hour blast in southern Russia were students going to classes.
Security is being further tightened ahead of Sunday's parliamentary poll.
A minister suggested Chechen rebels were behind the bombing near the town of Yessentuki but Chechnya's separatist leadership in exile denied this.
"We condemn all terror acts and acts of violence... directed against the civilian population," the separatists' foreign ministry said on the rebel website Kavkaz Centre.
The entourage of the Chechen rebel president, Aslan Maskhadov, has disowned suicide attacks in the past which sometimes were later claimed by Chechen warlords nominally allied to his cause.
President Putin said the attack was an attempt to destabilise Russia in the run-up to the elections.
Russian Justice Minister Yuri Chaika earlier suggested evidence at the scene pointed to "Chechen terrorism" as a possible theory for the blast which took place in a province which borders the war-ravaged region.
Investigators believe a male suicide attacker and three female accomplices carried out the attack which ripped a packed railway carriage in two as it was travelling through the southern region of Stavropol.
The remains of the suspected bomber were found with grenades still attached to his legs, the head of Russian Federal Security Service, Nikolai Patrushev, said.
He said two of the women had left a large bag in the carriage shortly before the blast and leapt from the train at the last moment.
The third woman - who was reportedly directing the operation - was seriously injured and unlikely to survive, Mr Patrushev added.
Thirty-five people died at the scene of the blast, reports say.
Many of the bodies have yet to be identified but an incomplete list of the dead released by the authorities shows that most were in their late teens or early twenties.
RECENT BLASTS IN RUSSIA
15 Sept: At least two killed at Russian security HQ in Magas, Ingushetia
3 Sept: At least four killed on Mineralniye Vody train
25 Aug: At least three killed at Krasnodar bus stops
1 Aug: 50 killed at Mozdok hospital
5 July: Chechen suicide bombers kill at least 14 at a rock concert near Moscow
Russian officials said they were students on their way to colleges of higher education in the nearby town of Pyatigorsk.
The casualties are being treated at hospitals in Yessentuki, Pyatigorsk and Kislovodsk.
President Putin promised that the federal government and regional authorities would "do their utmost to help the blast victims".
"The criminals will achieve nothing," he said as Stavropol declared Monday a day of mourning.
Messages of condolences have flooded in from the United States, Germany, France, Britain and other Western states, as well as the ex-Soviet republic of Ukraine.
"The United States strongly condemns this terrorist act and all acts of terrorism," White House spokesman Scott McClellan said in a statement.
"We stand with the Russian people in their fight against terrorism," the statement said.
Russian Interior Minister Boris Gryzlov vowed to track down the "beasts" behind the explosion, promising that the ground would "burn under their feet".
Shock and bitterness
Doctors treating the casualties said most had no memory of the blast which came after the train had left Yessentuki station, just before 0740 local time (0440 GMT) on Friday.
"No one could tell us what had happened," said one in Yessentuki.
"All they can recall is being thrown about the carriage. The shock waves hurled them against the walls."
There was bitterness, too, among local people that the bombing had not been prevented after two explosions on the same rail service in September which left six people dead.
"People are dying by the dozens and they [the security forces] are doing nothing," said one woman at the scene.