Noisy marchers distrustful of communist China's intentions for Taiwan walked for hours along roads in Taichung in the centre of the export-reliant, self-ruled island China claims as its own.
Taiwan negotiator P.K. Chiang and China's Chen Yunlin will meet in Taichung on Tuesday for more talks on the proposed Economic Cooperation Framework Agreement (ECFA), a pact aimed at slashing import tariffs and opening the banking sector that should be signed next year.
Tuesday's talks will be the fourth round since China-friendly Taiwan President Ma Ying-jeou took office last year.
We're opposed to the secret meeting, the non-transparent meeting between China and Taiwan, because it could bring steep losses to Taiwan, said protester Ho Shih-sen, 59, a retiree in Taichung. We could take a big hit.
Organisers said 100,000 people attended the march. Local police put the figure at 10,000.
The march, organised by the anti-China opposition Democratic Progressive Party, is expected to be followed by more protests this week, including at the airport when Chen arrives on Monday.
Protests during China-Taiwan talks in Taipei last year sparked rioting in which police and demonstrators were injured.
Among the protesters were hardliners who want Taiwan to declare formal independence from China. Some waved banners advocating one side, one country.Some feared the ECFA would lead to a flood of competing goods from China, calling for open talks and for Ma to step down.
Ma Ying-jeou, our president, wants to sign ECFA but hasn't received public approval for it, said protester Charles Lee, president of an environmental group in southern Taiwan.
We're worried he will sell us out.
Protesters also feared the ECFA would allow Chinese competition for professional qualifications and jobs in Taiwan.
The government has pledged to exclude any labour deal from the trade pact.
Also on Tuesday's agenda is a deal to avoid double taxation while lowering both corporate and personal income taxes, and incentives for Taiwan investors in China as well as foreign firms based on one side but active on the other.
China has claimed sovereignty over Taiwan since 1949, when Mao Zedong's forces won the Chinese civil war and Chiang Kai-shek's Nationalists fled to the island. Beijing has vowed to bring Taiwan under its rule, by force if necessary.
(Editing by Paul Tait)