Asia and Pacific leaders began arriving in Thailand on Friday for a summit focusing on the global financial crisis, as police installed razor wire across the entrance of the venue to hold back protesters.
Earlier, several thousand "red-shirt" supporters of ousted prime minister Thaksin Shinawatra pushed past four police and military checkpoints, before stopping just outside hotel in Pattaya where the weekend summit was to be held.
Protesters shouted slogans against Thai Prime Minister Abhisit Vejjajiva, who has resisted calls to step down and order early elections.
Security has been stepped up for the summit and related meetings, which brings together the 10-member Association of Southeast Asian Nations (ASEAN) with New Zealand, Australia, China, Japan, South Korea and India.
New Zealand Prime Minister John Key arrives in Thailand on Saturday for the summit and will also visit China for talks with President Hu Jintao and Premier Wen Jiabao.
A Radio New Zealand political reporter says there is a heavy police and military presence around the Pattaya venue.
Deputy Thai Prime Minister Suthep Thaugsuban told media that the summit will carry on. He said the police chief was talking with protest leaders and "all means necessary" would be taken to reopen a road leading to the venue.
About 7,000 police and military have been sent to the resort town. The government also declared Friday a public holiday to try to reduce the effect of the protests.
However, local media report that a state of emergency could be declared if protests get out of hand.
Television footage on Friday showed riot police and red-shirted protesters angrily pushing each other on a road leading to the hotel in the first major confrontation since mass protests began on Wednesday in Bangkok.
In a televised address on Thursday, Mr Abhisit also said he would not give in to demands to step down made by supporters of Mr Thaksin, who was ousted in a military coup in 2006 and lives in self-imposed exile.
Supporters of Mr Thaksin say Mr Abhisit, elected by parliament in December, is a pawn of the army.
Financial crisis on agenda
On Friday, ASEAN leaders from Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam were meeting to discuss how to move forward with their political and security community.
The two-day summit, which begins on Saturday, will will focus on regional responses to the global financial crisis and will be briefed by the heads of the World Bank, the International Monetary Fund, the United Nations and the Asian Development Bank.
It was postponed last year after protesters seized Bangkok's domestic and international airports.