The leader of Taiwan's opposition Nationalist Party, Lien Chan, has arrived in China for an historic eight-day, four-city tour.
Mr Lien says he hopes to encourage reconciliation between the two sides
Mr Lien is due to hold talks with Chinese President Hu Jintao on Friday.
It will be the first meeting between Nationalist and Communist Party leaders since the Nationalists fled to Taiwan after losing China's civil war in 1949.
Critics say Mr Lien, who steps down this year, sees the visit as a chance to ensure his political legacy.
Taiwan's ruling Democratic Progressive Party, which favours independence from China, says Beijing is using the visit to try and split public opinion in Taiwan.
The Nationalists, also known as the Kuomintang, were for decades implacably opposed to dealing with China's Communists. But since they lost power in 2000 they have increasingly favoured closer ties, especially for business reasons.
Mr Lien arrived in the central Chinese city of Nanjing, the Nationalists' former capital.
"Taipei and Nanjing are not too distant, but it still took 60 years to come here. It certainly took too long to make the journey," Mr Lien told the official welcoming party.
The BBC's Caroline Gluck, in Taipei, said there were chaotic scenes and scuffles inside the airport before Mr Lien left as supporters and opponents of his visit clashed.
At least two people were injured and had to be taken to hospital in the fighting, in which firecrackers, eggs, water bottles and bamboo sticks were used as weapons.
Ruled by separate governments since end of Chinese civil war in 1949
China considers the island part of its territory
China has offered a "one country, two systems" solution, like Hong Kong
Most people in Taiwan support status quo
Opponents chanted that Mr Lien was "selling Taiwan out" by going on the trip, our correspondent said.
The two groups had to be separated by heavily armed police, but Mr Lien avoided the trouble entirely having been taken into the airport via a VIP entrance.
Our correspondent says the already strained relations between China and Taiwan have worsened in the wake of China's recently passed anti-secession law, which legalises the use of force against Taiwan if the island declares independence.
Mr Lien, who was born in China, has called his trip to the mainland a journey of peace - saying he hopes it will help the two sides work towards reconciliation after more than half a century of conflict.
"I hope the journey will contribute in whatever small way towards mutual assistance, mutual concern, and the creation of a win-win situation," he said.
Mr Lien is being accompanied by senior officials from the Nationalist, or Kuomintang, party, and by more than 100 reporters.
The government, initially strongly critical, is now cautiously backing the trip.
President Chen Shui-bian has said visits to China by opposition leaders would have his blessing if they acted according to the law and did not sign agreements with Beijing without government authorisation.
Another opposition leader, James Soong, head of the pro-unification People First Party, has also accepted an invitation to visit the mainland, and is expected to travel to China next month.