Arrests were made in Colombo late Thursday after veteran politician Ranil Wickremesinghe was sworn in as Sri Lanka's new president.
Protesters were seen in police custody near the president's office.
One was filmed on the ground surrounded by paramedics and clutching his head.
Sri Lankans have taken to the streets for months to demand their top leaders step down to take responsibility for economic chaos that has left the nation's 22 million people struggling with shortages of essentials, including medicine, fuel and food.
Wickremesinghe, a six-time prime minister, has never held the top job.
But he easily won the secret ballot of lawmakers Wednesday to finish the term of former President Gotabaya Rajapaksa, who fled the country after protesters stormed his residence last week and resigned.
His appointment received mixed reactions, with some supporters lighting firecrackers while protesters continued to demand that he resign.