Pro-election activists have vowed to visit Prime Minister Prayut Chan-o-cha at Government House on the fourth anniversary of the military coup this month if their demands for an early vote are not met.
The demonstrators, led by Rangsiman Rome, Nuttaa Mahattana and Sirawith "Ja New" Seritiwat, gathered at the Tha Phrachan campus of Thammasat University in Bangkok on Saturday evening after a month-long hiatus for the Songkran holidays.
About 600 Chana Songkram policemen were deployed to provide security. They expected 350 people would join the rally.
Mr Rangsiman repeated the three demands of his group: the general election must be held in November, the National Council for Peace and Order (NCPO) must resign to ensure the polls are free and fair, and the military must stop backing the NCPO and return to the barracks.
Gen Prayut has said the election would be held in early 2019.
"If the demands are not met soon, we'll go visit Gen Prayut at Government House to declare our stand at 10am on May 22," Mr Rangsiman said, referring to the anniversary of the military coup four years ago.
The demonstrators pointed to numerous broken promises of elections to explain the reasons for their rally.
"We were asked why we have to come out. Our answer is that since heading the government, Gen Prayut has promised to hold the election dozens of times but kept delaying it. That's why we still need to fight," said Mr Rangsiman.
"We believe the NCPO is afraid of us. So if you don't do the right thing, the people, who are like a knife at its throat, will come out. We're certain we on the right path and this is people's success."
Mr Sirawith said that as long as the NCPO is in power, a fair election can never be held.
"The door is closed and it's high time the NCPO unlocked political parties so they can get ready for the election," he said.
The demonstrators later released "Yutnocchio" balloons to make a point. Images of the prime minister with a long Pinocchio nose have become a main symbol of their campaign.
The rally included a "censure debate" as the demonstrators feel the National Legislative Assembly has not done its duty to scrutinise the regime and check the powers of the NCPO.
Mr Sirawith told Thai media earlier that the no-confidence debate would focus on the regime's attempt to perpetuate its power by mobilising former MPs and influential figures in the provinces to support a military-aligned party that would back Gen Prayut as prime minister after the election.
They said mobile cabinet meetings, such as one planned in Buri Ram on Monday, were part of the junta's strategy to cement its political base. Gen Prayut has insisted the Buri Ram trip is non-political and is intended to find out what kind of assistance local people need.
In a related development, Internet Law Reform Dialogue (iLaw), a non-government organisation, on Saturday began accepting signatures to propose a bill aimed at revoking 35 announcements and orders by the NCPO. Spokesman Yingcheep Atchanont said the announcements violate human rights and democratic principles.
They include those banning political gatherings of five people or more and those restricting the media.
iLaw aims to draw 10,000 people to sign up so it can propose the bill, as allowed by the constitution.
