* Protesters blockade meeting venue
* Anti-government "red shirts" clash with pro- "reds"
* Organisers say summit will go ahead
By John Ruwitch
PATTAYA, Thailand, April 11 (Reuters) - Anti-government protesters in Thailand disrupted an Asian summit on Saturday, forcing the cancellation of meetings by the foreign ministers of Japan, South Korea and China.
Leaders of the Association of Southeast Asian Nations (ASEAN) were scheduled to sign an investment agreement with China, but red-shirted supporters of ousted Thai prime minister Thaksin Shinawatra blockaded the venue, forcing the meeting to be delayed.
"The ASEAN-China meeting has been delayed because of attempts to block delegations and leaders,"Government spokesman Panitan Wattanayagorn told reporters. "They have confirmed they are still interested to attend. We wish to ensure their safety and we expect the meetings to resume soon."
The meeting between the Japanese, Chinese and South Korean foreign ministers would have been their first opportunity to discuss last Sunday's launch of a North Korean rocket widely viewed as a long-range missile test.
Protesters blocked the entrance to the meeting venue, and Japanese Foreign Minister Hirofumi Nakasone was unable to reach it from his nearby hotel, a Japanese foreign ministry official said.
"The trilateral meeting has been cancelled," the Japanese foreign ministry official said, "But the Japanese foreign minister has spoken by telephone separately with his South Korean and Chinese counterparts."
The East Asia Summit brings together the 10 members of the Association of Southeast Asian Nations (ASEAN) and China, Japan, South Korea, India, Australia and New Zealand for discussions about trade, economic issues and regional security.
The full summit itself takes place on Sunday and will focus on responses to the global financial crisis. On Saturday, ASEAN leaders were due to hold a series of mini-summits with their North Asia counterparts.
RED SHIRTS VS BLUE SHIRTS
Hundreds of troops were guarding entrances to the Royal Cliff Beach Resort in Pattaya, about 150 km (90 miles) south of Bangkok, but "red shirts" were already massing at the sprawling resort complex, their numbers swelled since Friday.
"There have been clashes in the periphery of the meeting venue, but this won't affect security of the meeting," Panitan, the government spokesman said. "A few people have been injured, just two or three, with minor injuries."
More security headaches arrived in the form of "blue shirts", a new group of pro-government supporters making their debut at the summit, as Thailand's deep political divisions play out on an international stage.
"The blue shirts are people who we believe are concerned about the meetings," Panitan said. "They want them to continue, and they want them to continue peacefully."
But Reuters photographers said the "blue shirts", wearing balaclavas around their heads to keep from being identified, were armed with truncheons, bricks and slingshots and were clashing with the "red shirts" on streets leading to the Royal Cliff. Smoke bombs were thrown.
On Friday, at least 2,000 of "red shirts" broke through roadblocks manned by police and soldiers, pushing aside police vans and buses and even driving away a fire truck, before confronting soldiers at the summit venue.
They only dispersed after an official from the secretariat of the Association of Southeast Asian Nations (ASEAN) came out to accept a letter from them.
The protesters say the government of Thai Prime Minister Abhisit Vejjajiva is illegitimate and not empowered to sign any summit agreement because he came to office in December through parliamentary defections they say were engineered by the army.
Thaksin, now in exile, was ousted in a 2006 coup. His reconstituted party, however, regained power after elections, which sparked months of protests last year that closed airports in Bangkok and took a huge toll on the economy.
The Chinese investment agreement due to be signed on Saturday is the final piece of a Free Trade Agreement that will create the world's largest free trade area, encompassing 1.9 billion people.
The ASEAN leaders were also scheduled to hold separate meetings with the leaders of Japan and South Korea and then with all three later on Saturday in the so-called ASEAN + 3 process.
On Friday, Southeast Asian foreign ministers "expressed concern" about tensions in Northeast Asia during a meeting on politics and security on Friday.
(Additional reporting by Isabel Reynolds and Martin Ruwitch; Writing by Bill Tarrant; Editing by David Fox)