BEIRUT – Syrian forces Friday fired on protesters holding the largest opposition marches yet in Aleppo, a sign of rising anti-regime sentiment in the country’s biggest city, which has largely remained supportive of President Bashar Assad throughout the 15-month uprising.

The head of the U.N. observer mission in Syria warned that neither his team nor armed action could solve the country’s crisis, and called on all sides to discuss a solution.

But the regime kept up its assaults on opposition areas and protests, while the head of Syria’s largest exile opposition group dismissed the U.N.’s plan as unrealistic.

Anti-regime protests in Aleppo have been growing since a raid on dormitories at Aleppo University killed four students and forced the temporary closure of the state-run school earlier this month.

The May 3 raid was an unusually violent incident for the northern city, a major economic hub, where business ties and large minority populations have kept most residents on the side of the regime — or at least unwilling to join the opposition.

On Thursday, some 15,000 students demonstrated outside the gates of Aleppo University in the presence of U.N. observers, before security forces broke up the protest.

Even bigger numbers took to the streets Friday. Aleppo activist Mohammad Saeed said it was city’s largest demonstration yet, with more than 10,000 people marching in the Salaheddine and al-Shaar districts and nearly as many more elsewhere in the city.

“The number of protesters is increasing every day,” Saeed said. He said several people were wounded when government forces fired tear gas and live ammunition to disperse the rallies.