Protesters say Mr Samak is protecting his predecessor
Thousands of people have demonstrated in Bangkok, defying warnings of a crackdown issued by Thai Prime Minister Samak Sundaravej.
Riot police were deployed, but the authorities did not carry out the PM's threats, saying force would not be used as long as the protest was peaceful.
Opposition activists accuse Mr Samak of changing the constitution to protect his predecessor, Thaksin Shinawatra.
Reports say the instability coincides with rumours of an imminent coup.
The demonstration was led by the People's Alliance for Democracy, the group which led mass protests in 2006 that led to the military coup that ousted Mr Thaksin.
On Saturday, some protesters wore helmets and masks, apparently to protect themselves in case tear gas was used.
There has been heightened political tension and demonstrations in recent weeks over the government's plans to revise the military-drafted constitution.
Riot police were on the streets but did not use force against protesters
The opposition says Mr Samak is a close ally of Mr Thaksin, who they believe has a hidden, anti-monarchy agenda.
It says Mr Samak is trying to change the military-backed constitution to hold on to power and to prevent Mr Thaksin from facing corruption-related charges.
On Friday, a Thai cabinet minister accused of insulting the king - which is taken very seriously in Thailand, where the king is a revered figure - announced his resignation in an attempt to calm political tensions.
Jakrapob Penkair, who will be charged next week, strongly denies insulting the king, but said he was stepping down to end rumours of a military coup which were prompted by the allegations.