If rebels seize control of Raqqa, it would be the first time an entire city had fallen into the hands of anti-Assad fighters. Rebels hold parts of several major Syrian cities - several neighborhoods in Aleppo, Homs and Deir el-Zour, as well as suburbs of the capital, Damascus. They also control large areas in the countryside, particularly in the north.
The rebel advances are a significant blow to Assad, although during the past week his forces have regained control of several villages and towns along a key highway near Aleppo International Airport.
The Britain-based Syrian Observatory for Human Rights said rebels are now in control of "large parts" of Raqqa, a city of around 500,000 on the Euphrates River, which flows through Syria into Iraq. A top police officer was killed and intelligence officers were detained, the group said.
Activists declared Raqqa "liberated" on opposition social media websites Monday. A photo posted on several pro-rebel Facebook pages showed people tearing down a huge poster of Assad and hitting it with their shoes. The activists said the picture was taken inside the feared Air Force Intelligence headquarters in Raqqa.
An amateur video posted online shows a large, bronze statue of Hafez Assad being torn to the ground by a rope tied around the statue's neck. The video shows people in the crowd taking photos and video on their cellphones as the statue crashes into the side of a fountain, while others fired guns into the air in celebration. People then rush to jump on the statue and hit it with their shoes.
"God be with the Free Army!" they shout.
The video appeared consistent with AP reporting.
Amir, an activist in Raqqa, said rebels were now in control of the city but that there were pockets of pro-regime elements still fighting.
The mood was euphoric in the city when residents and rebels toppled the statue in the main square, but "then the shelling began and everyone fled home," Amir said via Skype, with the crackle of gunfire audible in the background.
There were also airstrikes on the city and many casualties, he said, agreeing to give only his first name for security reasons.
Rebels have been making headway in Raqqa province for weeks, capturing the country's largest dam. On Sunday, anti-Assad fighters stormed the Raqqa central prison.
Earlier Monday, rebels launched an offensive to try to seize the military air base of Mannagh near the Turkish border and clashed with government forces at a historic mosque in the nation's largest city of Aleppo, activists said.
In Saudi Arabia, U.S. Secretary of State John Kerry and Saudi Foreign Minister Prince Saud al-Faisal together warned Assad that they will boost support to rebels unless he steps down.
Saudi Arabia has been one of the region's harshest critics of Assad's regime. In his discussions with Kerry, Saud said he stressed the importance of enabling the Syrian people to exercise their "legitimate right to defend itself against the regime's killing machine." Saud also complained that the Assad regime continues to get weapons from "third parties," a veiled reference to Russia and Iran, which have backed Assad through the conflict.
Kerry criticized Iran, Hezbollah and Russia by name for giving weapons to Assad's forces.
Monday's fighting came as a pro-government newspaper reported that opposition fighters killed 115 police and wounded another 50 in a battle Sunday over a police academy in the north. The daily Al-Watan reported that "terrorists committed a massacre" at the academy near Aleppo.
A government official, however, denied the report, and said instead that 23 government troops were killed and that seven were still missing. The official spoke on condition of anonymity because he was not authorized to brief the media.
The report came a day after the Observatory said the rebels seized the police school in Khan al-Asal after entering the sprawling government complex with captured tanks. The Observatory said the battle left at least 120 soldiers and 80 rebels dead.
The Syrian conflict started two years ago as a popular uprising against Assad's authoritarian rule, then turned into a full-blown civil war after the rebels took up arms to fight a government crackdown on dissent. The United Nations estimates that more than 70,000 people have been killed in the conflict.
Assad maintains his troops are fighting "terrorists" and Islamic extremists seeking to destroy Syria, and he accuses the West and its Gulf Arab allies of supporting them.
The Observatory said clashes also were raging inside Aleppo's landmark 12th century Umayyad Mosque, a UNESCO World Heritage Site in the walled Old City. The mosque was heavily damaged last year after a fire gutted the city's famed medieval market.
On Saturday, the Syrian army command said it captured areas in Aleppo opening a road between the government-controlled central city of Hama with Aleppo's international airport. The airport, the country's second largest, has been the target of a rebel offensive for weeks.
The Observatory said rebels on Monday destroyed the Assan bridge near the airport. An amateur video showed rebels blowing up the bridge, creating a thick black of smoke amid chants of "God is great."
The video appeared genuine and corresponded to other AP reporting on the events depicted.