A powerful blast was heard in Nalchik, the capital of Russia's North Caucasus republic of Kabardino-Balkaria, but no casualties were reported, a police source said on Friday.
The explosion was heard at 23.58 local time (20:58 GMT) on Thursday.
"The blast occurred when a police car was passing by," the source said. "The explosive device was probably planted under a manhole cover."
According to preliminary reports, no one was injured.
While Chechnya, under the leadership of former separatist fighter-turned-Kremlin ally Ramzan Kadyrov, has seen a marked reduction in militant attacks in recent years, violence has erupted in the neighbouring republics of Ingushetia, Dagestan and Kabardino-Balkaria.