UN observers have dampening effect on Syrian violence, Ban Ki-moon says Security forces open fire to break up protests in Aleppo, the opposition says Video purports to show thousands in a regime stronghold calling for al-Assad's ouster The U.N. chief says observers in Syria are dampening the violence (CNN) -- Syrian forces opened fire Friday on anti-government demonstrators in Aleppo, the opposition claimed, a day after thousands purportedly took to the streets calling for an end to the regime in a city widely considered a stronghold of President Bashar al-Assad. Security forces were attempting to break up a demonstration that began after dawn prayers in the mixed, upper middle class city in northern Syria, according to the Local Coordination Committees of Syria, an opposition network that counts casualties and organizes anti-government protests. While there have been a number of demonstrations in Aleppo since the Syrian uprising began, the number of people publicly calling for al-Assad's ouster has increased significantly. It culminated this week with thousands of predominantly university students demonstrating, according to opposition groups, who posted videos of the protest outside Aleppo University on YouTube. The size of the recent protests raise questions about whether al-Assad is losing his grip on the country's largest city where he has enjoyed support. The increase in demonstrations in Aleppo comes at a critical time for both sides who are vying for international support in the conflict that began in March 2011 with protests calling for political reforms and quickly devolved into an armed uprising following a brutal crackdown by al-Assad's forces. The opposition took to social media to urge Syrians Friday across the country to take the streets in solidarity with the Aleppo demonstrators. Al-Assad has dismissed the opposition as foreign-backed terrorists bent on destabilizing the government. Opposition groups, including members of the rebel army, say al-Assad's government has been trying to hamper their efforts by accusing them falsely of links to terrorism. There were no immediate reports of casualties in Aleppo on Friday, though the LCC said two of the 34 people killed across Syria Thursday were from Aleppo. There were also reports of security forces shelling the beleaguered opposition stronghold of Rastan in Homs province, the LCC said. The reports of violence have cast severe doubts on the success of a peace plan brokered by U.N. special envoy Kofi Annan. Even so, U.N. Secretary-General Ban Ki-moon said Thursday that 260 observers of 300 planned were already deployed in Syria to monitor the progress of the peace plan. But Ban said recent attacks targeting the observers are "alarming and surprising." There were attacks against monitors, twice. Therefore, we're trying our best efforts to protect civilian population," he said. The deployment of monitors has some dampening effect. The number of [instances of] violence has reduced but not enough. Not all the violence has stopped. So we will continue, as much as we can, to protect the civilian population. A video posted Thursday on YouTube purported to show thousands of anti-government protesters outside Aleppo University, where men in blue helmets could be seen. The U.N. observers wear blue helmets. Meanwhile, another video surfaced on YouTube that purports to show Lebanon's military arresting a Syrian activist at a hospital in Tripoli where he was being treated for wounds sustained when Lebanese forces attempted to break up clashes between pro- and anti-Syrian regime groups. CNN can not independently confirm the authenticity of the videos. In recent days, violence has spilled over into Lebanon and Turkey, where thousands of Syrians have fled. At least one person was killed and an undetermined number were wounded Thursday in renewed clashes in the northern Lebanese city of Tripoli between factions supporting and opposing the uprising in Syria, Lebanon's National News Agency reported. The United Nations estimates that at least 9,000 people have died in the 14 months of conflict, while opposition groups put the death toll at more than 11,000. CNN's Amir Ahmed, Hamdi Alkhshali and Richard Roth contributed to this report.