Thai protest leader Suthep Thaugsuban says he has given prime minister Yingluck Shinawatra two days to "return power to the people".
Mr Suthep said a meeting with the prime minister was held under the auspices of the military and there was no compromise. He did not say what action would follow if the ultimatum were not heeded.
Demonstrations against the government began last Sunday.
The protesters say Ms Yingluck's administration is controlled by her brother, exiled ex-leader Thaksin Shinawatra, and they want to replace it with a "People's Council".
Mr Suthep said he met the prime minister in a secret location in the presence of the army, navy and air force commanders.
"There was no negotiation and no compromise," Mr Suthep was quoted as saying.
"I told Yingluck that this is the only and last time I see her until power is handed over to the people.
"There will be no bargaining and it must be finished in two days."
The BBC reports some 30,000 protesters earlier gathered at about eight sites, including Government House, television stations and the police headquarters.
A message from Mr Suthep has been broadcast by almost all channels in Thailand, in which he called a general strike for government employees for Monday.
On Friday Ms Yingluck ruled out early elections, telling the BBC that the country was not calm enough for polls. She repeated her call for negotiations to resolve the crisis.
Until Saturday the protests were largely peaceful, but four people have been killed since then.
The BBC reports the protests are the worst since 2010, when red-shirt Thaksin supporters occupied parts of the capital for two months. More than 90 people, mostly civilians, were killed.
Thaksin was overthrown by the military in September 2006 and his party was later banned.
Ms Yingluck's Pheu Thai party won a general election in July 2011. However, anti-government demonstrations began last month.