Stanislav Listyev was heading into the station at Sennaya square around 2.30 p.m. local time when the blast occurred.
"At that moment, I felt an explosion wave underneath, everything was filled with smoke, people started panicking. So the trains stopped and almost immediately the evacuation started," he tells CNN.
He said he thought the explosion happened between stations because of the location of the smoke.
"I think that explosion happened in the tunnel between the stations. The smoke was coming out of there. There was nothing on the station itself, everything was fine," Listyev says.
"I didn't see any injured people. No one was injured on Sennaya square."
The explosion in St. Petersburg took place in a tunnel between two metro stations, Russian state media reports, citing officials.
The blast occurred in a metro car that was traveling in the tunnel between the Sennaya Ploschad [square] and Tekhnologichesky Institute stations.
The St. Petersburg Metro is made up of five lines and 67 stations. Opened in November 1955, it transports 2.3 million people a day and has more than 1,600 train cars.(Source: CNN)