The protesters - civil-rights activists, union members and others - chanted, waved signs and cheered as speakers portrayed the mandatory photo ID requirement as part of a cynical effort by Republicans to suppress voters turnout and gain an advantage in a presidential election year.
"They know that, when people don't vote, they win. ... We're going to stop it and stand at the borders of Pennsylvania and say 'everybody in America has a right to vote,'" said Pennsylvania AFL-CIO President Rick Bloomingdale.
Hilary O. Shelton, senior vice president for advocacy at the National Association for the Advancement of Colored People, noted that even supporters of the law - one of the toughest in the nation - cannot cite any cases of voter impersonation in Pennsylvania.
"It is a solution looking for a problem," Shelton told the crowd gathered on the Capitol steps around a 25-foot inflatable replica of the Liberty Bell.
Similar protests are scheduled Wednesday in Philadelphia, Pittsburgh and Allentown.
The rally came one day before the state Commonwealth Court is set to open a hearing in Harrisburg on a lawsuit seeking to block the law from taking effect on Nov. 6, Election Day.
Civil-rights groups representing the 10 Pennsylvania voters who are the plaintiffs contend that the law is unconstitutional and would disenfranchise many poor people, senior citizens and minorities, who are less than likely to have the required photo IDs.
Supporters say the law is simply an extra layer of protection against voter fraud, although the state attorney general's office joined the plaintiffs in signing a stipulation that says neither side is aware of any incidents of in-person fraud.
The law was passed earlier this year by the Republican-controlled Legislature with no Democratic votes and signed in March by GOP Gov. Tom Corbett.
Secretary of State Carol Aichele, the state's top elections official, sought to quietly schedule a post-rally news conference while the protest was in progress, but word leaked out and a leader of the rally announced it over the public-address system.
About two dozen protesters tried to attend the news conference in a Senate meeting room, only to be blocked by Capitol police. Instead, they chanted and sang outside the room, nearly drowning out Aichele at times as she answered reporters' questions.
Inside the room, Aichele listed steps that her department has taken to ease difficulties facing some voters who lack a Pennsylvania driver's license or one of several other forms of ID that are acceptable under the law. They include a special State Department photo ID that will be made available to voters who have trouble obtaining birth certificate copies.
She expressed confidence that such initiatives and an intensive voter-education effort will ensure that all eligible voters can get valid IDs. The plaintiffs in the court case estimate at least 1 million of the 8.3 million registered voters currently lack them.
Several Democratic legislators who took part in the rally and were permitted to sit in on Aichele's news conference, followed up with one of their own.
They criticized authorities for limiting public access to the news conference and called for the law to be postponed, if not repealed.
"This is a process that's clearly not ready for implementation" this year, said Sen. Vincent Hughes, D-Philadelphia. "There's still too many kinks in this process."