Up to 70,000 people have rallied in Homs, activists say, as Arab League observers arrived in the Syrian flashpoint city.
The opposition claims tear gas was used on the crowd as it tried to enter a central square.
In pictures said to come from Homs, a man appears to challenge monitors, imploring their aid. In one video posted on YouTube, purportedly showing a street encounter with the mission, protesters shout: “We want international protection.”
The UN says at least 5,000 lives have been lost in Syria since the protest movement began.
More than 30 people were killed in Homs on Monday, according to activists. They say they do not want the monitors to be hoodwinked by being shown districts where life is relatively normal.
Some tanks were seen pulling out of the city before the observers arrived.
The monitors are in Syria to assess whether President Assad has halted a crackdown on the nine-month long anti-government protests.