The supporters of former Prime Minister Thaksin Shinawatra, deposed in a 2006 coup, say they will blockade a hotel where heads of state are to meet.
The new move follows three days of huge protests in the Thai capital, Bangkok, calling on Prime Minister Abhisit Vejjajiva to call new polls.
Mr Abhisit declared Friday a public holiday to help cope with the protests.
"Abhisit get out! If this government does not accept our demands we will not go home," shouted protesters gathered near the summit venue in Pattaya.
"We will go peacefully to the hotel, we will not damage anything, any government property. But we have to show the world that this government is not democratic," protest leader Arismun Pongreungrong told AFP news agency.
The comment was a reference to the failure of Mr Abhisit's Democratic Party to win elections.
He came to power when a court ruled the previous government led by allies of Mr Thaksin was illegal, after anti-Thaksin protesters had shut down the country's airport for eight days in December.
Summit
Mr Abhisit has refused demands to quit, and said protesters were free to express their views so long as they were peaceful.
But his hosting of a regional summit for the next three days has raised the stakes.
Security has been stepped up in Pattaya for the summit, which brings together the 10-member Association of Southeast Asian Nations (ASEAN) with China, Japan, South Korea, India, Australia and New Zealand.
"Nothing untoward will happen to the leaders. We will make sure of that. We have to make sure the meeting can go on," said Deputy Prime Minister Suthep Thaugsuban.
This weekend's summit will focus on regional responses to the worsening global economy and will hear briefings from the heads of the World Bank, the International Monetary Fund, the United Nations and the Asian Development Bank.
New tactics
The red-shirted protesters of the United Front for Democracy against Dictatorship (UDD) brought new tactics to their rallies on Thursday, blocking key junctions and enjoying the support of taxi drivers who blocked intersections.
The "political gridlock" as local media called it, affected thousands of commuters and prompted the government to threaten prosecution of protest leaders.
The UDD accuses Mr Abhisit's government of being a puppet of the military.
On Wednesday, they surrounded the home of an influential royal adviser in the capital.
They have accused Gen Prem Tinsulanonda of engineering the 2006 coup that removed Mr Thaksin from power.
