BEIRUT, Lebanon — Tens, and possibly hundreds, of thousands of people defied a continuing government crackdown to fill the streets of several Syrian cities on Friday, intent on showing visiting monitors from the Arab League the extent of opposition to President Bashar al-Assad.

As thousands marched in Idlib, Homs, Hama and in the suburbs of the capital, Damascus, violence flared at several of the rallies. By day’s end, activist groups said that more than two dozen people had been killed by security forces. The large crowds, while not unprecedented, underscored the resilience of the protest movement despite United Nations estimates that more than 5,000 people have died since opposition to the government galvanized in March.

A protester in Dara’a, who reported huge demonstrations, said: “We want to show the Arabs and the world that we are peaceful protesters, not criminals or armed gangs. The coming days and weeks will prove our statements, not the regime’s story.”

The government’s supporters also held rallies, according to witnesses and the Syrian state news agency, SANA, which posted photographs of large gatherings in Aleppo and Damascus. The news agency said the protesters were “demanding the Arab League observer mission to be credible and professional in conveying the facts of what the terrorist groups are perpetrating.”