Veteran party strategist Suthep Thaugsuban on Saturday night called on opponents of the blanket amnesty to gather at Samsen railway station at 10am Sunday and "be ready" for the next move.
"If the crowd is large enough and the conditions are right, we will upgrade our protest," the Surat Thani MP and former deputy premier told an estimated 20,000 people.
"Those who disagree with absolving the guilt of the wrongdoers should come and join our protest and listen to my announcement of the next move."
Mr Suthep had said earlier that the party would wait for the outcome of the Senate debate on the amnesty bill before deciding whether to take its protest to the next level.
The Samsen rally is one of three being held in the capital against the bill, which critics say would whitewash ousted former premier Thaksin Shinawatra. The Senate is expected to take up the bill on Nov 8 or 11.
Two more senior Democrats, Sathit Wongnongtoey and Supachai Srila, resigned on Saturday as party deputy secretaries in order to join the Samsen protest.
Meanwhile, concern is mounting in the business community about the potential economic impact of prolonged conflict over the deeply divisive amnesty bill.
Thapana Sirivadhanabhakdi, the president of Chang brewer Thai Beverage, said the country risked becoming "unplugged" if protests worsened. He was referring to Pheu Thai supporters' contention that a blanket amnesty would help "reset" the country.
Activist businessman Somkiat Homla-or, meanwhile, has called for a protest on Silom Road on Monday at lunchtime.
Addressing the Samsen rally, he said the Businessmen for Democracy Club and the Green Silom Business Group would ask people to blow whistles at 12.34pm Monday to demonstrate their opposition to the blanket amnesty. Participants are also being asked to wear green.
Mr Somkiat said the public should tell the government that it should not pass any law to clean up the deeds of wrongdoers.
"In the past, Silom people have shown strength to expel the governments including those led by Chavalit Yongchaiyudh and Thaksin Shinawatra. This time, I would like to ask Silom people to show the power again," he said.
Metropolitan police, meanwhile, were keeping a close watch at Samsen, as more protesters are expected over the weekend.
At the Uruphong intersection, police had demanded that organisers of the anti-government rally reopen the area for traffic by Saturday night.
But the leaders of the Network of Students and People for Reform of Thailand (NSPRT) are refusing to move, so police would gather evidence to seek arrest warrants for them from the court, said Pol Maj Gen Piya Uthayo, spokesman for the police peacekeeping command.
However, the spokesman said police had no plan to use force to disperse the protesters at Uruphong as some media had reported.
In other developments on Saturday, ASTV pulled the plug on its live broadcasts of the Samsen rally site, saying the Democrats were more interested in publicising their party than fighting for the cause.
ASTV was founded by Sondhi Limthongkul, one of the former core leaders of the yellow-shirt People's Alliance for Democracy (PAD).
Former PAD spokesman Parnthep Pourpongpan said the Democrats had failed to expand the scope of the rally beyond a single issue, which is the amnesty bill.
He said they should also be mobilising public anger against corruption and the Pheu Thai party's threat to the balance of power between executive and legislative branches.
"The Democrat Party has abandoned and shattered the expectation of the demonstrators," Mr Parnthep said.
As a result, he said, Mr Sondhi decided to end ASTV's live broadcasts from Samsen and cover the anti-government demonstration at the Uruphong intersection instead.
Democrat spokesman Chavanond Intarakomalyasut was unconcerned and said media outlets could choose what they wanted to cover.
Many other PAD supporters, however, said they backed the Democrats' stand and would join the Samsen rally because the amnesty bill was a major issue.
Don Muang resident Wanna Supawan, 66, said she sided with the PAD protests in 2008.
"I saw the television news [about the blanket amnesty bill]. I just can't ignore it anymore," she said. "It doesn't matter that the Democrats are leading this protest. I will support them."