Thailand's Senate on Friday delayed debate on a government-backed amnesty bill that has sparked mass protests, in a decision that could prolong political unrest.

Critics say the bill is aimed at bringing former premier Thaksin Shinawatra back from self-exile without serving jail time.

Thaksin, still adored by his mostly poor, rural supporters but distrusted by many members of the establishment, was convicted in absentia in 2008 of corruption - charges he said were politically motivated.

His sister, Yingluck Shinawatra, is now prime minister.

The lower house of parliament has passed the political amnesty bill and it has gone to the upper house Senate. But a group of 40 Senators boycotted Friday's session, which failed to attract the minimum number of 75 members needed for debate.

"Due to a lack of quorum, debate of this amnesty bill has been moved to Monday," said Senate Speaker Nikom Wairatpanij.

The decision could anger protest leaders who have put pressure on the Senate to reject the bill. Some protest leaders have called for the government's ouster.

"Time is up for this corrupt and evil government. We will chase it out," said Nitithorn Lamlua, a leader of the Network of Students and People for the Reform of Thailand group, the second biggest faction among the protesters.

Thaksin, a billionaire former telecoms tycoon, became the first leader in Thai history to win a parliamentary majority on its own, and formed the first elected government to serve a full term, after which it was re-elected.

But corruption scandals eroded his popularity among Bangkok's middle classes. That was compounded by royalist accusations that he was undermining the powerful monarchy, which he denied.

He fled from Thailand in 2008 and lives in Dubai but is widely believed to be pulling the strings of government from abroad. He remains a hugely divisive figure.