Lin Fei-fan, a protest organiser, estimated that 500,000 people had turned out in the biggest demonstration since the movement started.
Crowds dressed in black sat on one blocked boulevard, many carrying plastic or real sunflowers, the symbol of the protest movement, and wearing yellow ribbons that read "Fight for democracy, retract the service trade pact."
Several hundred mainly student protesters have been occupying Taiwan's Parliament building since March 18, supported by thousands outside the building.
Yesterday, Ma gave into students' demands to increase scrutiny of future pacts signed with China, but refused to withdraw the pact in question, saying it would deeply harm Taiwan's interests.
"I'm not against free trade, but the government should come up with policies to protect local industries before they open the door," said a protester, Philip Lihan, 30, a graphic designer in Taipei originally from Chiayi in southern Taiwan.