Syrian forces on Sunday carried out arrests in the western city of Hama, an opposition stronghold, amid the sound of heavy gunfire, an anti-government activist said.
The arrests came two days after some 300,000 protesters gathered in Hama in the largest demonstration yet in a three-month-old uprising against President Bashar Assad.
The protest carried important symbolism for the anti-government movement: in 1982, Assad's late father and predecessor, Hafez, crushed a rebellion in the city by Syrian members of the pan-Arab Muslim Brotherhood movement, killing thousands.
Sunday's arrests took place near Hama's sports stadium, said Lebanon-based Syrian activist Rami Nakhleh, who coordinates information from a loose network of activists in Syria.
Nakhleh said he knew of five arrests so far, but believes more were taken into custody.
He said activists in Hama told him intelligence officials handed notices to many youths suspected of participating in demonstrations, demanding they visit local security centers for interrogation. The activists said the youths were so far refusing to go.
Syrian troops had withdrawn from Hama after killing 65 people there in a crackdown on demonstrators June 1. There had not been a noticeable security presence in the city until Sunday's arrests.
But after the large demonstration on Friday, the Syrian president fired Hama's governor, Ahmed Abdul-Aziz.
Syria's state-run news agency did not say why the governor was dismissed. Some activists said they feared Abdul-Aziz, viewed as sympathetic to the demonstrators, was dismissed to give freer rein to the security forces in the city.
"There's a lot of fear there," said Syrian activist Ammar Qurabi, who is also monitoring movement around Hama.
Activists say security forces have killed more than 1,400 people across Syria in the uprising. The government has disputed that toll and says armed gangs are conducting an insurrection.
Also Sunday, the Swiss government said it froze 27 million Swiss francs ($31.8 million) linked to senior Syrian officials. A spokeswoman for Switzerland's State Secretariat for Economic Affairs, Antje Baertschi, said the assets were identified as part of the sanctions imposed against Assad and 22 other officials.
Switzerland has taken similar measures to freeze assets of other Arab leaders facing demonstrations against their rule.