At least 12 people have been reported killed in two explosions on the outskirts of Makhachkala, capital of Russia's restive Dagestan region.
The double blast late on Thursday targeted a police checkpoint on a road leading out of the city.
Medical and police officials said more were injured in the blasts.
Dagestan and neighbouring Russian republics have been plagued by violence in recent years between Islamist militants and Russian security forces.
The attack began when police stopped a vehicle on the road to the city of Astrakhan to check the driver's identity papers. The car exploded.
Less than 20 minutes later, a second blast went off, this time in a van packed with explosives. It caused further casualties.
"Human body fragments are scattered at the checkpoint. There are a lot of them there," a local police source told the Russian Interfax news agency.
Most of the dead are believed to be police officers and members of the emergency services.
The construction of the bombs and how they were set off was still under investigation, Interior Ministry spokesman Vyacheslav Gasanov told the Associated Press news agency on Thursday.
It is unclear who carried out the attacks, says the BBC's Steve Rosenberg in Moscow, but in Dagestan there are almost daily clashes between the security forces and Islamist insurgents and criminal gangs.
Dagestan, a predominantly Muslim, multi-ethnic republic which borders Chechnya, has seen some of the worst militant violence in the North Caucasus in recent years.