State police Inspector Gene Adamczyk said a judge ordered the Capitol reopened and authorities did so despite safety concerns.
Many of the protesters chanted "Whose house? Our house!" and stomped as they ringed the Rotunda.
Police earlier arrested eight people who tried to run into the Senate while it was in session.
Gov. Rick Snyder and Republican legislative leaders said today they would press for quick approval of right-to-work legislation, an announcement that set off a confrontation inside the Capitol when police used chemical spray to subdue protesters trying to rush the Senate floor.
State Police Inspector Gene Adamczyk said eight people were arrested for resisting and obstructing when they tried to push past two troopers guarding the Senate door. He said the troopers used chemical spray after the people refused to obey orders to stop.
The Capitol was temporarily closed because of concerns for the safety of people and the building. Adamczyk estimated about 2,500 visitors were at the Capitol.
So-called right-to-work measures generally prohibit requiring unions from collecting fees from nonunion employees, which opponents say drains unions of money and weakens their ability to bargain for good wages and benefits. Supporters insist it would boost the economy and job creation.
Should it become law in Michigan, it would give the right-to-work movement its strongest foothold yet in the Rust Belt region, where organized labor already has suffered several body blows.
After repeatedly insisting during his first two years in office that right-to-work was not on his agenda, Snyder reversed course Thursday, a month after voters defeated a ballot initiative that would have barred such measures under the state constitution.
"This is all about taking care of the hard-working workers in Michigan, being pro-worker and giving them freedom to make choices," Snyder said during a news conference with House Speaker Jase Bolger and Senate Minority Leader Randy Richardville, both fellow Republicans.
"The goal isn't to divide Michigan, it is to bring Michigan together," Snyder said.
But the decision to push forward in the waning days of the Legislature's lame-duck session infuriated outnumbered Democrats, who resorted to parliamentary maneuvers to slow action but were powerless to block the bills that were expected to be introduced Thursday.
Hundreds of chanting, whistle-blowing union activists packed the state Capitol rotunda and hallways, chanting slogans such as "Union buster" and "Right-to-work has got to go."