Both regions have proved crucial to the persistence of the uprising, the gravest challenge yet to the more than four decades of rule by the Assad family. Protests over the arrest of teenagers in Dara’a, a poor town in a drought-stricken agricultural area, soon galvanized nationwide demonstrations. Unrest has been particularly pronounced in the capital’s suburbs, and the Syrian government has sought to stanch its spread to Damascus.

Since the uprising began, Insan said that it had documented 2,434 arrests across the country and was still trying to verify the fate of at least 5,000 others.

“This is just what we know,” said Wissam Tarif, the group’s executive director.

In one high-profile arrest on Monday, he said security forces in the capital arrested Diana Jawabra, an outspoken critic of the siege of Dara’a, her hometown. She resisted and was forced into a car at gunpoint, Mr. Tarif said, citing witnesses. Ms. Jawabra, 39, had been trying to arrange a relief convoy, departing Tuesday, to the town, whose plight has prompted solidarity protests in other regions of Syria and in neighboring Jordan.

Residents of Dara’a had long seethed under the government’s heavy-handed control, in particular the sway of Atef Najib, a cousin of Mr. Assad’s in charge of security in the region, who became the subject of sanctions by the Obama administration last week. After the protests erupted in March, activists declared parts of the town liberated. That ended April 25, when the government sent tanks, soldiers and security forces into the town.

Electricity and phone lines were cut, as was water. Residents have complained of shortages of medicine, food and baby formula. For days, they said, no one was willing to leave their homes for fear of snipers occupying rooftops across the town.