The sister of ousted Prime Minister Thaksin Shinawatra announced an agreement on Monday to form a five-party coalition government after her Pheu Thai Party won a landslide victory in Thailand’s parliamentary elections.

Ms. Yingluck Shinawatra, whose brother was ousted from the prime minister post by a 2006 military coup, is set to become Thailand’s first female leader after a vote that marks a significant political comeback for Mr. Thaksin.

Ms. Yingluck, whose Pheu Thai Party already has won a majority of 265 seats in the 500-seat lower house of parliament under preliminary results of Sunday’s polling, announced an agreement that would boost her coalition to 299 seats.

That accord came unusually quickly for Thai politics, where hard bargaining usually takes place over allocation of Cabinet seats. The pact should gain Ms. Yingluck’s government-to-be some stability, especially if legal challenges under electoral law force some of her party’s lawmakers from their positions.

The Democrat Party, which has led a coalition government for more than three years, will be in opposition.

Earlier, Mr. Yingluck acknowledged huge challenges in reconciling the divided country, after an election victory seen as a rebuke of the military-backed establishment that ousted her brother.

The large mandate will likely boost Thailand’s stability in the short term -- the Thai stock market rose sharply Monday morning -- and reduce the chance of intervention by the coup-prone military.

The victory comes one year after the government crushed protests by Thaksin supporters with a bloody crackdown that culminated some of the worst violence here in 20 years and ended with parts of the capital ablaze in a wave of arson attacks allegedly carried out by fleeing protesters.

In a late night victory speech in Bangkok on Sunday, Ms. Yingluck said- “I don’t like to say that Pheu Thai has won, but I’d rather say the people have given the Pheu Thai party and myself a chance to serve them.”

“There’s still a lot of work to be done in the future, in terms of the well-being of the people and for the country’s unity and reconciliation,” Ms. Yingluck said.

On Monday, incumbent Prime Minister Abhisit Vejjajiva resigned as leader of the outgoing ruling party, Democrat Party spokesman Buranaj Smutharaks told The Associated Press. The Democrats won 159 seats.

The photogenic Ms. Yingluck is widely considered the proxy of her brother, who has called her “my clone.” Mr. Thaksin, who was ousted as prime minister after being accused of corruption and showing disrespect to the nation’s much-revered king, was barred from politics in 2007 and convicted on graft charges the next year. He lives in exile in Dubai.

His overthrow touched off a schism between the country’s haves and long-silent have-nots that continues to this day. The struggle pits the marginalized rural poor who hailed Mr. Thaksin’s populism against an elite establishment bent on defending the status quo that sees Mr. Thaksin as a corrupt autocrat.

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