Yingluck's Pheu Thai party dominated the country's general election, which took place Sunday. The official results have not been released, but with more than 90 percent of votes counted Monday, the Pheu Thai party had won 265 seats in the country's 500-member parliament.
"When compared with the result of (the) election in 2007 with this year's election, we have less MPs," Abhisit said via the Thai news agency MCOT. "I think that I need to take this responsibility, so today I decided to resign from the leader position of the (Democratic) party, and I will let the party choose a new leader in 90 days."
Yingluck was poised to become the new prime minister, five years after her older brother, former Prime Minister Thaksin Shinawatra, was ousted in a military coup.
"The first thing I want to do is help people on their economic situation," Yingluck said Sunday.
Thai stock jumped more than 3%, or 33 points, at opening on Monday following the Pheu Thai party's election win.
The election had more than a 70% turnout rate, the country's election commission said.
"We congratulate the people of the Kingdom of Thailand, our long-time friend and ally, for their robust participation in the July 3rd parliamentary elections," U.S. State Department spokeswoman Victoria Nuland said in a statement.
Thaksin, Yingluck's brother, is one of Thailand's most polarizing political figures.
Two years after the coup in 2006, he left the country after being convicted on conflict of interest charges -- accusations that he still denies.
Yingluck's critics worry she will simply do her brother's bidding -- something she has denied.
Before she even gave her victory speech, her brother shared his comments from exile in Dubai.
"Well, I would tell them that I really want to go back, but I will wait for the right moment and the right situation," Thaksin told reporters.
The Pheu Thai party remains fiercely behind Thaksin and wants him to return.
But the so-called "Yellow Shirts," a group that formed to oust Thaksin from power, has said it will do whatever it can to stop that from happening.
Yingluck said she won't give her brother favorable treatment.
"I can't do anything special for my brother," she said, adding she will follow the "rule of law."
With about 47 million eligible voters in Thailand, Sunday's balloting was held to decide Thailand's first general election since 2007, an election that many hope will bring an end to years of unrest between two political factions that climaxed last year with protests that turned deadly.
"There is a lot more hard work to do in the future for the well-being of our sisters and brothers, the people of Thailand," Yingluck said Sunday. "There are many things to accomplish to make reconciliation possible, paving the way for a solid foundation for a flourishing nation."
Tensions between the Democratic Party and the Pheu Thai party, which reflect deep divisions within Thai society, erupted last year, with protests against Abhisit's government leading to a military crackdown. More than 90 people were killed and hundreds were injured.
After the riots, the Thai government pledged to work toward a process of national reconciliation to heal class and political divisions, though the divide between the two groups remains wide.
But average Thai voters were more concerned with economic issues, wanting their leaders to shrink the gap between what they earn and the skyrocketing cost of living.
"Free education is good, care for elderly is also good. In fact every parties' policies are all good, the question is if they would ever implement them." Banorn Achriyawatkul said as she worked outside a polling station.
After being laid off from her job as a secretary, the mother of four is now a food vendor in the streets, trying to make ends meet.
Despite the animosity between them, the two major parties have made very similar promises to the people: a better economy, free education, and a major increase in the minimum wage -- exactly what many voters wanted to hear.
But analysts say the extravagant programs will cost more than Thailand can afford.
Supong Limtanakool of Bangkok University's Center for Strategic Studies said both parties made big promises they just can't keep.
"It will be something that we have to spend somewhere between an additional 1.5 trillion baht to 7.5 trillion baht (49 billion dollars to 244 billion dollars) with all the extravagant programs, which is five times the national budget. ... I mean, we'll be broke in one year," he said.
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CNN's Benjamin Gottlieb contributed to this report.
From Sara Sidner and Kocha Olarn, CNN
