Gunfire and sporadic explosions have rattled Daraa after the Syrian army sent in more tanks and reinforcements as part of a widening crackdown.
In addition to the unrest in the city where the uprising began more than five weeks ago, security forces are conducting sweeping arrests and raids elsewhere in the country.
The crackdown came as opposition figures said their "massive grassroots revolution" will break the regime unless president Bashar Assad leads a transition to democracy.
A statement from an umbrella group of opposition activists in Syria and abroad called the National Initiative for Change said a democratic transition will "safeguard the nation from falling into a period of violence, chaos and civil war".
The statement added: "If the Syrian president does not wish to be recorded in history as a leader of this transition period, there is no alternative left for Syrians except to move forward along the same path as did the Tunisians, Egyptians and Libyans before them."
A vigorous crackdown since mid-March has killed more than 400 people, with 120 dead over the weekend, but the repression has only emboldened protesters, who started their revolt with calls for modest reforms but are now increasingly demanding Assad's downfall.
Mustafa Osso, who cited eyewitnesses on the ground in Daraa, said new troops arrived early on Wednesday from military bases in the area. Another rights activist, Ammar Qurabi, said security and armed forces have detained more than 400 people in Daraa since a major operation began in the city two days ago.
The army sent tanks into Daraa, 80 miles south of Damascus, on Monday and there have been reports of shooting and raids there and in areas across the country ever since.
Witnesses and human rights activists said the army also deployed tanks around the Damascus suburb of Douma and the coastal city of Banias, where there have been large demonstrations in recent weeks.
One Douma resident said security agents were going house-to-house, carrying lists of wanted people and conducting raids. If they did not find the person they were looking for, they took his relatives into custody, the resident said.