Thousands of demonstrators are on the streets of Taipei, venting their anger at a trade deal with China, to be signed on Tuesday.
The agreement will cut tariffs on exports and loosen curbs on investment.
Supporters say it will boost Taiwan's economy but critics fear it could pave the way for a Chinese takeover of the island.
Taiwan and China split after the civil war in 1949 but Beijing still regards the island as a renegade province.
Leaders of the opposition Democratic Progressive Party are calling for Taiwan to hold a referendum on the pact, known as the Economic Cooperation Framework Agreement.
Job loss fears
Taiwan's government has said the deal is crucial to keep its exports competitive with those of China's other trade partners.
But the opposition DPP say the deal could be the first step in a Chinese political takeover. Most Taiwanese resist the idea of unification with the Communist mainland.
The opposition also warns that a flood of cheap Chinese imports could lead to factory closures and the loss of jobs.
Despite decades of animosity, the two neighbours have become major trading partners, says the BBC's Cindy Sui in Taipei.
China has offered what are considered unusually generous terms, cutting tariffs on 539 Taiwanese products entering its market. These include car parts, petrochemicals and fruit.
In contrast, only 267 Chinese products would benefit from reduced tariffs on the Taiwanese side.
China would be able to invest in some Taiwanese service industries, while Taiwan would gain access to sectors such as computer services and airline maintenance in China.
The government of President Ma Ying-Jeo has made closer ties with China one of its leading policies.
Its aim is to reduce tensions across the Taiwan Strait, a flashpoint for decades since 1949.
China still has more than 1,000 missiles targeting the island as a warning against declaring formal independence.
Opinion polls indicate that a majority of Taiwanese back the trade agreement.
It could open the way for Taiwan to sign free-trade deals with other countries, something Beijing has resisted in the past.