Riot police have clashed with protesters armed with sticks and bottles outside the summit of 21 Pacific Rim leaders in South Korea.
Police used water cannon to force back the thousands of demonstrators.
Farmers joined the march in the city of Busan to protest at plans to allow more foreign rice imports to South Korea.
The two-day summit of Asia-Pacific Economic Co-operation (Apec) members is expected to focus on stalled global trade talks.
Some members have criticised the EU as an obstacle to progress and free trade.
Thousands of slogan-chanting protesters gathered outside the summit, shouting: "No to Bush, No to Apec. No to foreign rice imports."
Two farmers are reported to have killed themselves by drinking herbicide, leaving suicide notes blaming the rice market plans.
Police surrounding the summit were forced to block the demonstrators with shipping containers and drive them back with water cannon. More than 30,000 riot police are on duty in the port city and a naval blockade is protecting the seafront conference centre where the leaders are gathering.
The 21 Apec leaders are set to push strongly for a breakthrough on global trade talks at next month's World Trade Organisation talks.
They are also expected to agree a joint action plan on fighting bird flu.
Meanwhile, China announced a free trade agreement with Chile - its first with a Latin American country.
The deal - allowing Chileans to send most of their exports to China tariff-free - is an example of growing economic relations between China and Latin America, say analysts.
Apec is expected to issue a joint statement at the end of the summit on Saturday, calling for concessions on farm imports at next month's WTO meeting in Hong Kong.
But at the start of the summit South Korean Foreign Minister Ban Ki-moon suggested the European Union could be more flexible.
"The leaders here are sending out a very strong message that they will do their best for the successful negotiation of the talks," he said. "They are basically saying that now the ball is in Europe's court."
A draft copy of the statement is expected to say: "We call for the breaking of the current impasse in agriculture negotiations."
The combined 21 Apec economies, from the US to Japan, China and Australia, represent 57% of the world economy.
Other issues are also expected to occupy the agendas of Apec leaders:
- South Korea plans to discuss the withdrawal of some of its 3,000 troops serving in Iraq.
- President Bush calls on South East Asian countries to put more pressure on Burma to reform.
- Analysts say Japanese Prime Minister Junichiro Koizumi faces a frosty reception from China and South Korea over his visits to the Yasukuni war shrine, which honours convicted war criminals as well as war dead.
- Japan and Peru are also tussling over the fate of ex-Peruvian President Alberto Fujimori, arrested in Chile and wanted in Peru on charges of corruption and human rights abuse.
President Bush has already met his Russian counterpart Vladimir Putin for their fifth meeting this year.
They discussed the Iranian and North Korean nuclear issue as well as the situation in Syria, said Russian presidential aide Sergey Prikhodko.