Mr Abhisit said more details would be revealed in a TV address on Sunday
Thailand's PM Abhisit Vejjajiva has rejected a new, conditional offer by anti-government red-shirts to end weeks of protests in return for early polls.
The red-shirts said on Friday they wanted parliament dissolved within a month, a change from previous calls for immediate dissolution.
They also called for an investigation into recent violence. Clashes two weeks ago with police left 25 people dead.
The red-shirts have been occupying parts of Bangkok for six weeks.
"Because they use violence and intimidation I cannot accept this," said Mr Abhisit.
Leaders of the red-shirt movement said they now feared a government crackdown to forcibly disperse them from their Bangkok encampment.
An army spokesman had earlier said that what he called terrorists infiltrating the crowds would be suppressed, but the conditions were not yet right for such an operation because of the presence of so many innocent protestors, including women and children.
Hopes of a possible peaceful way out of the crisis appear to have been short-lived, says the BBC's Bangkok correspondent Rachel Harvey, and tensions in the city are rising once more.
The red-shirt protesters had previously demanded the government step down immediately and hold elections.
"The 30-day ultimatum is not an issue," said Mr Abhisit. "The dissolution [of parliament] must be done for the benefit of the entire country, not just for the red-shirts, and it must be done at the right time."
14 Mar: Red-shirts converge on Bangkok, hold first big rally, occupy government district
16 Mar: Protesters splash their own blood at Government House
30 Mar: A round of talks with the government ends in deadlock
3 Apr: Red-shirts occupy Bangkok shopping district
7 Apr: PM Abhisit orders state of emergency
10 Apr: Troops try to clear protesters; 25 people are killed and hundreds injured
22 Apr: Grenade blasts kill one and injure 85 near protest hub; each side blames the other
24 Apr: Red-shirts offer talks if elections held in one month
23 Apr: Prime minister rejects offer
More details would be revealed on Sunday, he added. "Tomorrow everything will become more clear when I and the army chief will jointly appear on my weekly television address."
He has already offered to hold elections by the end of 2010, a year ahead of schedule.
Red Shirt leader Nattawut Saikua said his group was pulling out of talks with the government as Mr Abhisit's refusal to compromise made it pointless to continue negotiations.
"These negotiations will stop," he was quoted as saying by AP. "We will not talk anymore."
Many of the protesters are supporters of former Prime Minister Thaksin Shinawatra, who was ousted in a military coup in 2006.
They say Mr Abhisit's government is illegitimate because it came to power in 2008 by securing parliament's backing after the judiciary dissolved the previous pro-Thaksin government.
Their encampment, in Bangkok's shopping district, stretches south to the business district, where protesters have built large barricades.
A failed crackdown on 10 April left 25 people dead and hundreds injured. Since then tensions have risen steadily, with the Thai capital increasingly choked by the protests.
On Thursday, one person was killed and more than 80 injured in grenade attacks near their barricades.
The government said grenades launched from a southern corner of Bangkok's Lumpini Park, which is behind the red-shirts' main barricade, had caused the explosions.