Police originally arrested 155 people, but some were freed because they were able to present valid travel documents, said Colonel Worawat Amornwiwat, a spokesman for the immigration service.
"These people are minority Tamils fleeing war in their country and seeking asylum through the UN. But they came to Thailand with tourist visas, allowing them to stay only 60 days, and they overstayed," he said.
Some of those arrested on Monday have papers showing that they have already applied to the UN for refugee status, police said.
Worawat said that authorities would contact the Sri Lankan embassy in order to send back those charged with breaching their visas.He added that there was no provision for Sri Lankan refugees in Thailand, despite their registration with the United Nations. No one from the UN refugee agency was immediately available to comment.
Activists based in Canada, which has recently sought to change its immigration laws in response to the arrival of boats carrying hundreds of Tamil refugees, said those arrested in Thailand were genuinely in need of protection.
The Tamil refugees in Thailand may be persecuted if they return home and are "likely to face torture," the Canadian Tamil Congress said."We are very disturbed by the arrests as it includes genuine refugees seeking international protection," the group said in a statement.
Sri Lanka's crackdown on the Tamil Tigers, which ended a lengthy civil war last year, has resulted in a flood of people seeking asylum in other countries.