Abhisit made overtures to the poor rural masses that were the foundation of support for Thaksin Shinawatra, the former prime minister ousted by a military coup in 2006 but whom protesters claim was still behind the government succeeded by Abhisit.
"I will work for all Thai people, both those who voted for me and against me," said the country's third prime minister in four months. "Today, our country must be united."
"I am well aware that the political situation is abnormal," he said, speaking shortly after he was sworn in. "My first job is to end a failed political system."
He said his government would retain populist policies - including cheap credit and healthcare - implemented under Thaksin.
Abhisit, 44, was voted by parliament to be the country's prime minister on Monday after a court dissolved the People Power party leading the previous government, which was packed with Thaksin's allies.
His appointment is expected to bring at least a brief period of calm, although there have already been small and sporadic protests by supporters of the previous government.