TAIPEI, Sept 15 (Reuters) - About 250,000 people demonstrated in two Taiwan cities on Saturday to back the island's doomed efforts at securing United Nations membership, a move condemned by rival Beijing and rejected by ally Washington.

Some 150,000 people, including President Chen Shui-bian, marched through the southern port city of Kaohsiung in pro-U.N. green shirts and waving flags. Political opposition forces in Taichung meanwhile marshalled at least 100,000 people.

"The biggest thing is for the United Nations and the United States to notice that this U.N. effort is not just something Chen Shui-bian is doing," said Kaohsiung demonstrator Wang Chun-kai, 35, a businessman from the nearby city of Tainan.

Government officials say they know the U.N. bid will fail, prompting speculation that the ruling Democratic Progressive Party (DPP) has used it to solidify a long-term agenda of greater independence from China by stirring anger at home.

The United Nations is expected to reject the bid on Tuesday.

"The U.N. bid, of course, has absolutely nothing to do with getting into the U.N.," said Ralph Cossa, president of the U.S.-based think tank Pacific Forum CSIS.

"I think it is mostly tied into the Taiwan identity issue and the DPP's efforts to lock future administrations into this mindset."

The DPP plans to hold a referendum alongside Taiwan's presidential election next March on whether the democratically ruled island should seek U.N. membership as a new member under the name Taiwan.

The referendum, which has raised tensions between Taiwan and its key ally the United States as Washington seeks better China ties, is expected to pass if it makes the tough administrative grade of qualifying for the ballot.

Foreign powers will note the referendum, if it passes, but not waiver in their support for China, political analysts say.

China sounded off on Saturday. Shanghai, a likely target of Taiwan missiles in the event of conflict, held a major air raid drill, a sign China still views war as a possibility.

Repeated attempts by Taiwan to join the United Nations under its formal title, the Republic of China, have failed in the past decade, most recently in July. The United Nations ousted Taipei in favour of Beijing in 1971.

China's status as a veto-wielding member of the U.N. Security Council means the island stands no chance of entry.

China has seen self-ruled Taiwan as part of its territory rather than as a separate country since the island broke away from China in 1949 when Mao Zedong's Communists came to power.

For Washington, now increasingly engaged with Beijing on economic and regional security issues, the island is needlessly and dangerously provoking China by pressing for U.N. membership.

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