Fifteen opposition legislators were inside the chamber, along with several pro-government lawmakers, but no one was injured in early Wednesday's incident.
Ecuadorean TV broadcast images of two policemen firing tear gas into the darkened chamber and lawmakers rushing out through a side door. Some lawmakers were given oxygen after they left.
The session had ended at 1530 GMT on Tuesday, but about 40 lawmakers stayed inside.
Congress President Omar Quintana cut off electricity to try to force them out. By 0700 GMT, he authorised police to remove the 15 still there.
A few pro-government legislators, including Quintana, had stayed in the Congress to ensure that the opposition didn't hold an assembly while they were away.
The Congress had been debating attorney-general candidates, and the opposition was threatening to block a government-backed nominee.
One of the government-aligned parties, the populist PRIAN party, had said it would defect, threatening to give the opposition enough support to become the majority.
"We're no longer partners with Gutierrez," PRIAN party leader Alavaro Noboa said on Tuesday, referring to President Lucio Gutierrez.
"We're no longer partners with Gutierrez"
PRIAN party leader
Quintana abruptly ended the legislative session on Tuesday. Without the PRIAN lawmakers on its side, the pro-government bloc was reduced from 53 to 44 seats in the 100-seat unicameral Congress.
PRIAN Congressman Kenneth Carrera said on Wednesday that his party had no intention of joining forces with its sworn enemy, the conservative Social Christian party, to provide the opposition a clear majority.
He said his party broke with Gutierrez exclusively over the government's list of candidates for the attorney-general post.
Of three possible appointees, Gutierrez most strongly backed attorney Jorge Lopez, who topped the list.
Last year saw protests against
Gutierrez's economic policies
Congress reconvened on Wednesday afternoon and voted 60-0, with 38 abstentions, to reject all three of Gutierrez's candidates.
Gutierrez and a government-aligned majority bloc in Congress have faced a broad political backlash since December, when pro-government lawmakers replaced 27 of Ecuador's 31 Supreme Court judges in a simple majority vote - a clear violation of Ecuador's Constitution.
The judges were replaced with magistrates mostly affiliated with the pro-Gutierrez congressional bloc.
In the face of mounting criticism, Gutierrez has proposed a referendum to ask Ecuadorean voters to approve a constitutional amendment to have a new Supreme Court chosen by an independent panel made up of lawyers, unions and other sectors of civil society.