Opponents of the bill say it is designed to bring former Prime Minister Thaksin Shinawatra back from overseas exile. After being overthrown in a 2006 military coup, Thaksin fled into self-imposed exile in 2008 to avoid serving a two-year prison sentence on a corruption conviction.
His sister and the current prime minister, Yingluck Shinawatra, said in a televised address today that the amnesty could solve the country's longstanding political divisions.
More than 90 people were killed in 2010 during a crackdown on rallies in the heart of Bangkok by pro-Thaksin "Red Shirts" who were demanding that a Democrat Party-led government resign.
The original draft of the bill, approved in principle by the House of Representatives in August, did not extend the amnesty to leaders of the pro- and anti-Thaksin groups, but a committee in mid-October changed the bill to include them.
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The Senate is expected to debate the bill next Monday. If it rejects the legislation, the lower house can wait 180 days to pass it again and forward it to the king for formal approval.
Yingluck urged the senators to consider the bill "on the basis of reconciliation and compassion."
She said she believes the lower house will accept the Senate's decision, suggesting that her Pheu Thai party will not push the legislation further if the Senate rejects it.
Yingluck's speech prompted Pheu Thai to announce it would back down if the Senate rejects the legislation.
"What we want is, clearly, peace for the people. If we thought we were creating reconciliation but instead were making the conflict spiral, we are willing to respect the Senate's decision," Pheu Thai Secretary-General Phumtham Wechayachai said.
A group of senators said today they will reject the legislation.
The amended legislation has been criticized by various groups, including international rights organizations, critics of Thaksin, and even Red Shirt members who oppose immunity for those involved in the 2010 crackdown.