The ruling Democratic Progressive Party (DPP) hopes half a million people will eventually join a rally in the southern city of Kaohsiung to promote a referendum on applying to join the UN under the name "Taiwan."
As the march got under way, police said a crowd of around 100,000 had gathered while organisers put the number at close to 300,000.
DPP supporters, many wearing T-shirts printed with the slogan "UN for Taiwan," gathered in front of an arch with a huge "UN" sign on it.
"Join the UN, referendum protects Taiwan," the supporters shouted, pushing their fists into the air while waving placards and flags.
President Chen Shui-bian and DPP presidential candidate Frank Hsieh were expected to use a videolink to address overseas Taiwanese groups gathering outside the UN headquarters in New York, the party said.
The independence-leaning Chen has repeatedly vowed to press ahead with the planned vote despite fresh warnings from Beijing and Washington.
After talks in Sydney last week, US President George W. Bush and China's Hu Jintao voiced concerns over Taipei's plan and Hu warned that it could propel the region into a "possibly dangerous period."
Names and titles are highly sensitive in the row between Taiwan and China, which split after the end of a civil war in 1949.
The island, under its official name the Republic of China, lost its UN seat to China in 1971.
Its efforts to rejoin the world body using its official name have been repeatedly blocked by Beijing, which regards Taiwan as part of its territory awaiting reunification.
Separately, the main opposition Kuomintang (KMT) party said some 100,000 people joined a rally in the central city of Taichung headed by its presidential candidate Ma Ying-jeou. Police estimated a turnout of 50,000.
The KMT is proposing its own referendum on whether the island should push to join international organisations as the "Republic of China," its official title, or "Taiwan."
Ma and his running mate Vincent Siew appeared dressed in blue, the party's colour, and waved national flags appeal to grassroots supporters.
"We are seeking to return to the UN with a pragmatic and flexible approach. The DPP's proposal to join the UN under the name 'Taiwan' is unfeasible and it is an election ploy," Ma told the crowd.
In contrast to the carnival atmosphere in central and southern Taiwan, dozens of people scuffled with police outside the Presidential Office in Taipei, where they were gathered to oppose the planned referendum.
Some sprayed red paint on photographs of the president and on government banners promoting the UN bid in a show of anger.
Some 15 of Taiwan's 24 allies have proposed the island's membership application to the General Assembly, which will decide whether to discuss it when the annual session opens on September 18.
Last Mod: 15 Eylül 2007, 17:15