A suspected female suicide bomber has set off explosives on a bus in the southern Russian city of Volgograd, killing at least six people.
Officials believe the woman was from Dagestan in the North Caucasus and was the partner of an Islamist militant.
The blast, which happened just after 14:00 (10:00 GMT), also injured more than 30 people, some of them seriously.
An Islamist insurgency in the North Caucasus region has led to many attacks there in recent years.
The BBC's Steve Rosenberg in Moscow says the bombing has raised fears that militant groups may be planning to step up attacks in Russia in the run-up to the Winter Olympics in the Black Sea resort city of Sochi next February.
It is believed there were about 40 people on board the bus.
Our correspondent says that all buses in Volgograd have been ordered back to their depots to be searched for any sign of explosives.
One man whose daughter survived the explosion told Moscow Echo radio: "It was a powerful explosion - a huge blast. There were lots of students on the bus."
Another man, who was driving behind the bus, told Rossiya-24 television: "There was a blast - a bang - all the glass flew out of the windows. The cloud of smoke quickly dissipated and then I saw people start to fall out and run out to escape the bus. It was a horrible sight."
Earlier reports had suggested the blast might have been caused by an exploding gas canister.
Volgograd lies about 900km (560 miles) south of Moscow and 650km north of the North Caucasus.
Vladimir Markin, of the Investigative Committee - Russia's equivalent of the FBI - told the RIA Novosti news agency: "A criminal case has been opened under articles outlining terrorism, murder and the illegal use of firearms."
Mr Markin was later quoted by the Interfax agency as identifying the suspected suicide bomber as a woman from Dagestan.
He said: "According to preliminary information, the self-explosion was carried out by a 30-year-old Dagestani native, Naida Akhiyalova.
"According to investigators' information, the woman entered the bus at one of the bus stops and, almost right after that, the bomb went off. That is also confirmed by one of the passengers who survived."
In recent years, Russia has seen a number of attacks by women suicide bombers known as black widows, who are often related to Islamist militants and carry out attacks to avenge their deaths.
Female suicide bombers struck at two underground railway stations in Moscow in 2010, killing more than 35 people.
They were also believed responsible for explosions on two passenger jets at a Moscow airport in 2004 that killed about 90 people.
Separatists in Chechnya have fought two wars with Russian forces over the past two decades.
But the violence has spread across the North Caucasus in recent years, including to mainly-Muslim Ingushetia and Dagestan.
Hundreds of people, including members of the government and security services, have been killed.
President Vladimir Putin has stepped up security in the North Caucasus ahead of the 2014 Winter Olympics, which open in Sochi on 7 February.