At least 10 people are killed and more than 20 others hurt in a suspected suicide bombing in the Russian city of Volgograd, in what officials say is the second suicide attack there in less than two days.
The blast comes in less than 24 hours 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 900 km south of Moscow, 650 km north of the North Caucasus and 700 km north-east of Sochi.
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.
A spokesman for Russia’s Investigative Committee said both explosions were now being treated as acts of terrorism.
Sunday’s blast rocked Volgograd-1 station at around 12:45 at a time of year when millions of Russians are travelling to celebrate the New Year.
[youtube I3Sxyg7KBbM 650]