More than 2,000 anti-government protesters Monday blocked one of the busiest roads in downtown Bangkok, accusing police of bias in favour of supporters of ousted premier Thaksin Shinawatra.

Protesters from the so-called People's Alliance for Democracy (PAD) marched to the national police headquarters, in the middle of Bangkok's busiest retail and business district.

They have rallied for nearly seven weeks to call for the current government to step down, accusing Prime Minister Samak Sundaravej of acting as Thaksin's puppet.

Wearing yellow shirts in honour of Thailand's king, the cheering activists rallied in the street outside the police compound, snarling traffic in a city already notorious for its gridlock.

"We know there are good police and bad police. We want to support the good police, but for the bad police who help politicians, don't you feel ashamed?" said Somsak Kosaisuk, a PAD leader who spoke to the crowd from the back of a truck.

He accused the police of failing to aggressively investigate complaints that a former Cabinet minister and Thaksin aide had insulted King Bhumibol Adulyadej, a serious offence punishable by up to 15 years in prison.

Protests by PAD in early 2006 foreshadowed the coup later that year against Thaksin, who was accused of widespread corruption by the military.

The first corruption case against him is set to go to trial Tuesday.

The latest protest by the PAD comes as the group has begun calling for not just for Samak's resignation, but for dramatic changes to Thailand's system of government.

Sondhi Limthongkul on Friday told his supporters at a rally that Thailand needed a system of "new politics," which would appear to roll back many of the democratic gains of the last two decades.

He called for eliminating most elected seats in parliament, saying 70 per cent of the legislature should be appointed. He has not said who would appoint the lawmakers but the proposal harkens to the days when the military and the palace held sway over government.

"When there is an election, you have rights for only the four seconds it takes to cast your ballot. After that, the politicians have everything and no one will take care of you," he said.

"Politicians said, let's solve our problem in parliament. But we tried that and it didn't work," Sondhi said.