At least 10 people have been killed and more than 20 others hurt in a suspected suicide bombing on a trolleybus in the Russian city of Volgograd.
The blast comes a day after 17 people died in another suicide attack at the central station in the city.
Security has been tightened at railway stations and airports across Russia.
Moscow is concerned militant groups could be ramping up violence in the run-up to the 2014 winter Olympic Games in the city of Sochi.
The Olympic venue is close to Russia’s volatile north Caucasus region.
Volgograd lies about 900km (560 miles) south of Moscow, 650km north of the North Caucasus and 700km north-east of Sochi.
A spokesman for Russia’s Investigative Committee said both explosions were now being treated as acts of terrorism.Busy market
The latest explosion took place near a busy market in the city’s Dzerzhinsky district.
Maksim Akhmetov, a Russian TV reporter who was at the scene of the blast, said the trolleybus was packed with people going to work in the morning rush hour.
He described the scene as “terrible”, adding that the bus was “ravaged” and that there were “bodies everywhere, blood on the snow”.
The explosion removed much of the bus’s exterior and broke windows in nearby buildings.
The figures given for the number of dead and injured are still fluctuating – and a one-year-old child is said to be among the victims.
The first blast rocked Volgograd-1 station at around 12:45 (08:45 GMT) on Sunday, at a time of year when millions of Russians are travelling to celebrate the New Year.
A nearby security camera facing the station caught the moment of the blast, showing a bright orange flash behind the station’s main doors.
The explosion shattered windows and sent debris and plumes of smoke from the station entrance.
No group has yet said it was behind the blast.
An Islamist insurgency in the North Caucasus region has led to many attacks there in recent years. Insurgents have also attacked major Russian towns.
The attacks show that the bombers do not need to target Sochi directly to attract international attention – any part of Russia will do, says the BBC’s Daniel Sandford in Moscow.