Thailand's Senate convened a highly charged session Monday to determine the fate of an amnesty bill that could pave the way for the return of the country's divisive self-exiled former leader Thaksin Shinawatra.

Thousands of protesters rallied in several parts of Bangkok, raising concerns of renewed political violence after three years of relative calm. Nearly 7,000 police officers were deployed around the Parliament in the city's historic district, near the main protest site.

Critics led by the opposition Democrat Party say the bill was designed to whitewash the crimes of Thaksin, who is closely allied to the ruling party. They say the bill is an underhanded attempt by the government to pave the way for his return to Thailand, which he fled in 2008 to escape corruption charges.

Since it was passed by the lower house Nov. 1, the bill has set off demonstrations in Bangkok by both pro- and anti-government supporters.

"This could push the country's stability to the brink," The Bangkok Post said in a front-page editorial Monday, one of many newspapers that called for calm.

The protests have been peaceful, but more than a dozen Bangkok schools located near protest sites closed Monday or sent students home early, citing safety precautions and the inconvenience of stand-still traffic jams caused by the demonstrations.

Thaksin, whose sister Yingluck Shinawatra is now prime minister, retains wide support, especially from rural voters who gained from his populist policies. But he remains highly polarizing seven years after being ousted by a 2006 military coup over allegations of corruption and disrespect for Thailand's revered King Bhumibol Adulyadej.

Opposition to his return arouses fierce passions that sometimes have erupted into violence, most recently in 2010 when a military crackdown on pro-Thaksin supporters left about 90 dead.

The Senate speaker has vowed to reject the bill to defuse the tension on Bangkok's streets, and Yingluck has assured protesters that the ruling party will drop the legislation if the Senate strikes it down.

As Monday's debate got underway, the 149-seat Senate braced for a long day and night. Some 90 senators requested speaking time and each was granted a 10-minute limit, meaning the debate could last 15 hours.

"I cannot accept this bill," said Sen. Rosana Tositrakul, one of Thaksin's harshest critics. "Not only because the people have come out to oppose it, but because it is unconstitutional."

The original draft of the bill did not extend amnesty to the leaders of both the pro-Thaksin "Red Shirt" protests and the anti-Thaksin "Yellow Shirt" groups, but a House committee vote in mid-October changed the bill to include both. The last-minute change led to criticism that it was planned all along to apply to Thaksin.

The Senate debate coincided with another highly charged ruling on the unrelated matter of a territorial dispute with Cambodia, which has fueled nationalist passions on both sides for decades. The U.N.'s highest court was set to rule later Monday on which country owns a patch of land near a 1,000-year-old temple at their shared border.

Two issues have no connection but the government was concerned that protest leaders would use the temple ruling to stir up anti-government passions.