National television broadcast what it called the live swearing-in ceremony from the presidential palace in the capital, Damascus.
"Syrians, three years and four months ... have passed since some cried 'freedom,' " Assad said. "They wanted a revolution, but you have been the real revolutionaries."
He added: "Those who lost their way can now see clearly ... the monstrous faces have been unveiled, the mask of freedom and the revolution has fallen."
As we reported last month, Assad won the presidential elections, which were criticized as a sham by outside observers, with 87.7 percent of the vote.
Assad's regime has spent the past 3 1/2 years fighting a bloody civil war that erupted in the wake of the Arab Spring elsewhere in the region. But while other leaders such as Egypt's Hosni Mubarak were ousted, Assad remains firmly in power. The civil war, meanwhile, has created a massive humanitarian crisis, with more than a million Syrian refugees spread across the Middle East and Europe.
But as The Associated Press notes:
"Reflecting the security threat surrounding Assad, the inauguration ceremony was for the first time held at the presidential palace and not in the Syrian parliament as has been the tradition.
"Syrian TV announced Wednesday morning he would be sworn in at noon. His previous term in office was to expire on Thursday, and he had been widely expected to be sworn in then."