The car bombs filled with shrapnel went off shortly after midnight near the city's central square, with an interval of some 15-17 minutes, the police said. The second blast targeted the police convoy, which rushed to the scene of the first explosion, RIA Novosti reported. A police Lt Colonel was among the killed, NTV channel reported.
According to state-run Rossiya 24 news channel, up to 40 kg TNT equivalent explosives were used in the improvised explosive devices probably planted in the two parked cars or kept under them. A total of 60 people, including 44 policemen, were injured in the blast. Two persons, one of them a police officer, are in critical condition.
The republic of Daghestan bordering Chechnya has become one of the main sources of instability in the Caucasus region with frequent attacks on security forces, police and many major terror attacks in other parts of the country. Twin bomb blasts in Moscow metro in 2009 were also carried out by suicide bombers from this region.