Jatuporn Prompan on Sunday called for people on all sides to put aside conflict and band together to oust Prime Minister Prayut Chan-o-cha, describing him as a threat to the country and the centre of all problems.
The United Front for Democracy against Dictatorship (UDD) chairman was addressing a rally of about 500 people at Santiporn Park, near the May 6 heroes memorial, off Ratchadamnoen avenue. It began about 4pm.
Mr Jatuporn said the campaign would continue, with a daily rally at Santiporn Park calling for Gen Prayut's removal from office.
Among those in attendance were Pibhop Dhongchai, a former core member of the People's Alliance for Democracy (PAD), Veera Somkwamkid, secretary-general of the People's Anti-Corruption Network, Karoon Saingam, a former Buri Ram MP, Taikorn Polsuwan, a former leader of an anti-Thaksin group, and Saknarong Mongkol, a lecturer at Thammasat University's faculty of law.
Others included Chinawat Chankrachang, a member of the Ratsadon group, Payao Akkahad, mother of a nurse killed at Wat Pathumwanaram in 2010, and Suwich Sumanon, chairman of the Train Workers' Union.
The organiser of the rally was Adul Kheoboriboon, chairman of the committee of the relatives of May 992 heroes.
Three companies of crowd control police were on duty to ensure law and order.
Attendees took turns going up on the stage to speak. Mr Jatuporn went on stage at 7.35pm and made a closing speech.
He said he was not at the rally in the name of the UDD, because he did not want to get any organisation involved.
"During this time, all people must join hands to oust Gen Prayut, who is the country's present threat. If we do not sacrifice, Gen Prayut will stay on for another six years," Mr Jatuporn said.
Since taking power in the May 22, 2014 coup, Gen Prayut had become the centre of all problems and none of his promises had materialised, Mr Jatuporn said.
During the past 15 years, the country had been severely hurt by conflict and the only one who benefited from it was Gen Prayut.
"Today, the differences between the yellow and red shirts should be put aside. Instead, we should band together to remove Gen Prayut," he said.
"The 2017 constitution is Thailand's worst, and it is Gen Prayut who has benefited the most from it," Mr Jatuporn said.
He said there would be a daily rally calling for Gen Prayut's ouster at Santiporn Park.
"Tomorrow (April 5), we will rally again at this venue from 4pm to about 9pm. We will skip the rally on April 6 to make way for groups fighting for constitutional amendment. We will return on April 7," he said.
The rally was called off at 8.30pm.