The Thai Senate will reject an amnesty bill critics say is aimed at bringing back convicted former premier Thaksin Shinawatra from exile, the Senate Speaker said on Wednesday, a move that could defuse rising tension on the streets of Bangkok.

The bill is aimed at whitewashing crimes committed by all leaders involved in political unrest since 2004 and is backed by the ruling Puea Thai Party of Prime Minister Yingluck Shinawatra, Thaksin's sister.

"I reject this bill and will send it back to the lower house. We will not accept this amnesty and the Senate majority agrees with me," Senate Speaker Nikom Wairatpanij told Reuters.

Thousands of protesters have taken to the streets of Bangkok since Friday in protest at the bill, threatening to disrupt months of calm in a country scarred by bloody unrest in 2010.More than 5,000 students at Bangkok's prestigious Chulalongkorn University marched through the capital in protest against the bill on Tuesday as public outrage gathered momentum.

The bill will be debated in the Senate on Monday when it will need support from at least 76 of the 150 senators to pass. Critics say the bill is designed to allow Thaksin to return to Thailand without serving jail time after being found guilty in absentia in 2008 of corruption.

Nikom has worked for governments backed by Thaksin in the past and is widely thought to be supportive of the Puea Thai-led administration of his sister Yingluck.

Rights groups say the amnesty would absolve anyone accused of crimes connected to political violence, waive punishment for offenders and perpetuate a cycle of violence and culture of impunity.

In her first official remarks since the draft bill passed Thailand's lower house last week, Yingluck said on Tuesday she would leave the bill's fate in the hands of senators.

Some analysts read her remarks as a sign of retreat after Thaksin, who is widely believed to be pulling the strings of government from abroad, misjudged the political temperature.

"This is a sure signal that Thaksin wants to reverse and back out. Yingluck chose to act quickly and sent a strong message to the Senate before her party's image is left in tatters," Siripan Nogsuan Sawasdee, a political analyst at Chulalongkorn University, told Reuters.

Opposition leaders heading the protests in Bangkok have vowed to continue their occupation of the city's Democracy Monument area until the bill is thrown out.

"We cannot trust the government and their words until this law is withdrawn from parliament. We will stay until that happens," former premier Abhisit Vejjajiva, leader of the opposition Democrat Party, said in a speech to protesters at Democracy Monument.

If it becomes law, the amnesty would also whitewash charges against Thaksin's enemies, including Abhisit and his deputy, Suthep Thaugsuban, who were charged with murder for ordering a military crackdown on pro-Thaksin protesters in 2010. About 90 people were killed in the violence.