Thai protest leader Suthep Thaugsuban says he has met PM Yingluck Shinawatra and given her two days to “return power to the people”.
Mr Suthep said the meeting had been 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.
On the eighth day of action to unseat the PM, police fended off protesters who descended on key sites in Bangkok. Four people have died in the unrest.
Dozens have been injured.
The protesters had declared Sunday the decisive “V-Day” of what they termed a “people’s coup”.
They 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 had 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.”
Some 30,000 protesters had gathered earlier at about eight sites, police said, including Government House, television stations and the police headquarters.
Protesters did enter several TV stations to ensure a message from Mr Suthep was aired. It was broadcast by almost all of Thailand’s channels.
He called a general strike for government employees for Monday.
The BBC’s Jonathan Head in Bangkok says protesters had approached their targets in cheering, colourful columns, but could not get past the clouds of gas and concrete barricades.
Ms Yingluck had intended to give media interviews on Sunday at a Bangkok police complex but was forced to leave when protesters tried to break in.
A police spokesman, Piya Utayo, said security forces would be deployed to take back “government property” occupied by protesters in the past week.
However, many protesters remain on the streets and the weekend’s violence marks a sharp escalation of the unrest.
Deputy PM Pracha Promnok urged people in the capital to stay indoors from 22:00 to 05:00 local time (15:00-22:00 GMT) “so they will not become victims of provocateurs”.
Election ruled out
The worst violence occurred when students attacked vehicles bringing pro-government activists to a Bangkok stadium on Saturday.
Early on Sunday, pro-government “red shirt” leaders said they were ending their mass rally at the stadium to allow security forces to police rival demonstrations.
Up until Saturday the protests had been largely peaceful, with Ms Yingluck saying the government would only use minimum force.
Soldiers were then called in to help the riot police.
Our correspondent says military commanders have been reluctant to get involved but agreed to deploy the troops on condition they would carry no weapons and would stand behind riot police ringing the main government offices.
There is a high risk of greater violence if more supporters of Ms Yingluck – whose party enjoys strong backing outside Bangkok – try to come to the capital, our correspondent adds.
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.
Thailand is facing its largest protests since 2010, when thousands of red-shirt Thaksin supporters occupied key parts of the capital. More than 90 people, mostly civilian protesters, died over the course of the two-month sit-in.