Three Taiwanese opposition legislators on Tuesday started a 70-hour hunger strike to protest at what they term illegal moves by the ruling party to ratify a contentious trade agreement with China.

The Democratic Progressive Party (DPP) lawmakers made their move after a legislator from the ruling Kuomintang party announced on Monday that the trade pact had been approved by a joint committee.

The committee's approval was the first of three ratifications needed to pass the bill.

The China-sceptic DPP said the announcement was illegal, since the bill had not been deliberated for a single minute since the joint committee opened its session last Wednesday.

The committee has been stalled since the dispute sparked a brawl Monday between rival lawmakers.

But Kuomintang legislators insisted that the trade pact had been automatically passed by the joint committee, after it failed to be screened within the required three months due to opposition filibustering.

The three DPP legislators threatened to extend their protest fast if necessary.

Late on Tuesday around 200 students and activists broke through a security barrier and took over the main chamber in Taiwan's parliament, singing and dancing in protest against the trade agreement.

The group then attempted to barricade themselves inside the building using chairs and furniture from the chamber.

The DPP and the smaller but more radical Taiwan Solidarity Union also pledged to mobilise supporters when parliament holds a full session on Friday for the second reading of the trade bill.

The agreement signed last June is designed further to open up trade in services between the Taiwan and China, which split 65 years ago after a civil war.

The two opposition parties say it will hurt small service companies and damage the overall economy. They insist it be reviewed clause-by-clause, a demand rejected by the Kuomintang.

Under the deal China will open 80 service sectors to Taiwanese companies, while Taiwan will allow Chinese investment in 64 sectors.

The pact is one of the follow-up agreements to the sweeping Economic Cooperation Framework Agreement signed in 2010 to reduce trade barriers.

Cross-strait ties have improved markedly since President Ma Ying-jeou of the China-friendly Kuomintang came to power in 2008 pledging to strengthen trade and tourism links. He was re-elected in January 2012.

But China still considers Taiwan as part of its territory awaiting reunification -- by force if necessary.