The demonstrators, led by the People's Alliance for Democracy, accused police of protecting Thaksin and his loyalists against prosecution by stalling their investigations.

Thaksin was toppled in a bloodless military coup in 2006 following months of street protests led by the alliance. Critics accused the former leader of corruption and abuse of power.

Thaksin returned from exile earlier this year, vowing to clear his name in court.

More than 3,000 protesters gathered in front of the police headquarters in downtown Bangkok, near three of the country's most expensive shopping malls.

"I come to police headquarters to ask you why you have been sitting on the cases against Thaksin and his followers. These cases are moving nowhere," Sondhi Limthongkul, an alliance leader, shouted through a loudspeaker. "We are here to ask you to do your duty."