By Panarat Thepgumpanat
BANGKOK, Aug 7 (Reuters) - Thailand's parliament was due to debate a political amnesty bill on Wednesday as anti-government protesters marched to try to get it scrapped, saying it could let ex-premier Thaksin Shinawatra return from exile without having to serve a jail sentence.
Thaksin was ousted by the military in 2006 and found guilty of abuse of power, fleeing the country before the sentence was handed down. His sister, Yingluck Shinawatra, is now prime minister and critics says her government wants to find a way to get the conviction wiped so he can return a free man.
Opposition leader Abhisit Vejjajiva led a march of about 2,500 supporters towards parliament.
"It's not about organising a mob but about spreading the truth to the people, to recognise the dangers of this amnesty bill, that there are different hidden agendas," Abhisit told his Democrat Party's Blue Sky News television channel.
Thaksin, a former telecommunications tycoon, is hugely popular among poor urban and rural people and he led his party to two big election victories before he was ousted.
But he is mistrusted by the military-royalist establishment who say he is corrupt and authoritarian.
Abhisit was prime minister during violent unrest in 2010 when Thaksin's red-shirted supporters tried to force early elections. More than 90 people, both protesters and security forces, were killed and Abhisit has been charged with giving orders to use live ammunition that led to civilian deaths.
About 4,800 police have been mobilised for protests over the amnesty bill this week and an internal security act has been implemented to allow measures such as a ban on gatherings in the area around parliament.
Financial market analysts have worried about the prospect of renewed unrest in Thailand, especially as coup rumours have resurfaced, and foreigners have sold stocks in recent sessions.
But overall the Thai bourse has edged up this week and it was flat on Wednesday morning.
The amnesty bill, sponsored by a lawmaker from the pro-Thaksin ruling party and backed by the government, would scrap convictions or charges involving protesters active in the waves of political unrest since the 2006 coup.
The draft bill does not provide for an amnesty for leaders of the various bouts of unrest but opponents argue it could be altered to achieve that as it makes its way through parliament.
Specifically, they believe Thaksin's graft conviction could be done away with, allowing him to return home a free man.
Anti-Thaksin "yellow shirt" protesters helped undermine his government and two later governments that supported him in 2008. Their protests included occupying Bangkok's two main airports that year, halting flights for days.
They have decided against joining this week's rally and have generally kept a low profile since the July 2011 general election won convincingly by Yingluck. (Additional reporting by Apornrath Phoonphongphiphat and Pairat Temphairojana; Writing by Alan Raybould; Editing by Robert Birsel)