VLADIKAVKAZ, Russia?In their biggest attack in months, rebels set off explosives and opened fire on a police convoy, killing 21 officers only yards from police headquarters in the Chechen capital Thursday, an official said.

The ambush?a day after 11 Russian servicemen were killed and 13 wounded in attacks in the southern Shatoi region?represented a significant surge in rebel activity after months of small scale attacks. It also struck a blow to the fledgling Chechen police force that Moscow is trying to set up in the war torn republic.

Thursday’s attack appeared to have taken the Russian military by surprise. Military intelligence officers had predicted an upsurge in rebel activity in May?but had expected it in the rugged mountains of the south, not in the capital, the Chechen administration official said.

Five suspects in the attack were detained, authorities said.