The rallies showed that protesters would not be cowed by heavy military deployments, scores of killings and mass arrests across Syria in a brutal crackdown on the biggest popular challenge ever to 48 years of authoritarian Baath Party rule.
The official state news agency SANA said an "armed terrorist group" killed four soldiers and kidnapped two others in the city where Assad sent tanks and troops to crush resistance Monday.
The latest violence broke out after Friday prayers as thousands of people hit the streets across the country demanding Assad's removal and pledging support for the residents of Deraa, a city of 120,000 where the unrest originated on March 18.
"The people want the overthrow of the regime!" demonstrators chanted in many protests, witnesses said.
Defying violent repression which a Syrian rights group says has killed 500 people, more demonstrations flared in the central cities of Homs and Hama, Banias on the Mediterranean coast, Qamishly in eastern Syria and Harasta, a Damascus suburb.
Activists also reported protest rallies in Damascus itself.
Al Jazeera television aired footage from the village of Mahala near Deraa and from Banias and Homs. Protesters waved Syrian flags and banners saying: "No to the siege of Deraa," "A powerful country is the one whose people are free" and "We are preachers of freedom and peace, not saboteurs."
A witness in Deraa said Syrian forces fired live rounds at thousands of villagers who descended on the besieged city.
"They shot at people at the western gate of Deraa in the Yadoda area, almost three km (two miles) from the center of the city," he said. Another contacted by phone said he saw dozens who were injured being taken away by protesters in their cars.
Another resident said one man had also been killed by a sniper in the city.
Wissam Tarif, director of the Insan human rights group, said two people were killed in protests in Latakia. The killings could not be immediately confirmed. Foreign journalists have mostly been expelled from Syria.
A resident of Banias, Abdel Karim, said the demonstrations started from two mosques and were joined by nearby villagers.
The protests have drawn a cross section of Syrian society, which has been under monolithic Baath Party rule for the last 48 years, the last 41 led by the Assad clan. Bashar al-Assad preserved the autocratic Baathist political system he inherited in 2000 from his father, Hafez al-Assad, while the family expanded its control over Syria's struggling economy.
Inspired by the downfall of Egypt and Tunisia's leaders in popular uprisings, the upheaval could have regional repercussions since Syria, straddling Middle East political fault lines, is allied with Iran and backs the Hezbollah and Hamas militant movements, while holding intermittent indirect peace talks with Israel.
Before Friday's unrest started, a Syrian rights group said the army attack on Deraa has killed at least 50 civilians, and essential supplies in the city were running low.
Syria's exiled Muslim Brotherhood, which has been largely on the sidelines of the protests so far, called on Syrians to take to the streets Friday in support of Deraa.
It was the first time that the Brotherhood, ruthlessly crushed along with secular leftist movements during the 30 years of Hafez al-Assad's rule, had called directly for protests.
The group said government accusations that Islamists were behind the unrest were baseless and aimed at fomenting civil war and undermining demands for political freedoms.
But a Jordanian Islamist, Abu Muhammad al-Maqdisi, said that Muslims were obliged to join the protest and that the overthrow of Assad's minority Alawite rule would be a step toward implementing Sharia law in the mainly Sunni Muslim state.
Friday, the Muslim day of rest and prayers, has been the main opportunity for protesters to gather, challenging repeated warnings by the authorities not to demonstrate.
Security forces shot dead 120 protesters Friday April 22, according to a Syrian rights group, in the biggest protests Syria has seen since the uprising ignited in Deraa on March 18.
Three days later an army division under the control of Assad brother's Maher stormed into Deraa. That echoed their father's 1982 attack on Hama to crush an armed revolt led by the Muslim Brotherhood, killing up to 30,000 people.
In a sign of rare dissent within ruling circles, 200 members of the Baath Party resigned Wednesday in protest at the bloody crackdown.
A military official, quoted on the state SANA news agency, accused the media of exaggerating the death toll in the unrest, saying 78 security forces and 70 civilians had died.
The offensive intensified criticism of Assad in the West, which had taken steps to rehabilitate him in the last three years. Washington says it is considering tightening sanctions.
EU ambassadors were to meet in Brussels Friday to discuss the possibility of imposing sanctions against Syria, which could include asset freezes and travel restrictions on key officials.
"I'd expect a political signal toward sanctions but maybe not a decision yet," an EU diplomat said. Other EU measures against Syria could include freezing financial aid, which amounts to 43 million euros ($64 million) a year. (Writing by Dominic Evans; Additional reporting by Suleiman al-Khalidi in Amman, Yara Bayoumy; editing by Samia Nakhoul and Mark Heinrich)