MOSCOW - Russia beefed up security and mourned its dead on Tuesday as the toll from jarring successive-day suicide strikes in the run-up to the Sochi Winter Olympic Games rose to 33.
The southern city of Volgograd planned to bury the first victims of Sunday's bombing of the heartland hub's main railway station that killed 18 people and which officials have provisionally blamed on a young woman from the restive North Caucasus region of Dagestan.
A second suicide attack on a trolleybus packed with students on Monday that investigators pinned on a male assailant has now claimed 15 lives.
The blasts are Russia's deadliest since a suicide bombing at Moscow's Domodedovo airport that was claimed by Islamic insurgents from the North Caucasus killed 37 people in January 2011.
The latests blasts have laid bare the unchecked threat posed by guerrillas who have vowed to target civilians in a bid to undermine President Vladimir Putin as he welcomes world guests to Sochi for the Game's opening ceremony on February 7.
Putin has not spoken publically about the tragedies but is expected to address the nation during a midnight New Year address that will be watched closely for signs of how he intends to act next.
The powerful Russian leader issued a string of directives on Monday ordering security stepped up at public transit points across the nation and extra police deployed on the streets of Volgograd.
The city of one million - known as Stalingrad in the Soviet era and which is 690 kilometres (425 miles) northeast of Sochi - saw more than 5,000 law enforcement agents check traffic and inspect public transport on Tuesday morning.
"The operational headquarters of the Volgograd region has decided to call up extra reserves and enrol the maximum number of police and interior ministry soldiers possible," Interfax quoted regional security spokesman Andrei Pilipchuk as saying.
The nation's main Channel 1 television station led the news with images of helmeted soldiers with automatic rifles bursting into a Volgograd cafe and checking documents of dark-haired men who looked like they may have come from the mostly-Muslim North Caucasus.
Pilipchuk told the station that officers had arrested 87 people "who resisted police or did not have (their identity) documents".
Russia is already preparing to impose a "limited access" security cordon around Sochi from January 7 that will check all traffic and ban all non-resident cars from a wide area around the city.