The must-win victory will help Clinton stay in the race, despite trailing Barack Obama in pledged delegates.
Election officials reported huge turnout around the state, with voters queuing up at schools and firehouses from West Philadelphia to the working-class Pittsburgh suburb of Bellevue. The election, which ended with polls closing at 8 p.m., capped a furious six-week campaign by rival Senators Hillary Rodham Clinton and Barack Obama.
"I had to be here to vote, and I pray that Obama does make a big change in a very positive way," said Sarah Triplett, a black woman from Levittown who called herself "65-plus" and works with the disabled.
Penn State student Colin Cwalina, 19, wore his loyalties on his sleeve: He sported a "Hillary 2008" T-shirt at the polls in State College. But he was outnumbered by Obama supporters, including Penn State quarterbacks coach Jay Paterno - son of the school's famed football coach Joe Paterno - who reached out to prospective voters at the student union.
"To be honest, Barack Obama has a cult" following on campus, Cwalina said.
Nonpartisan voter-turnout groups cheered people in the University City section of Philadelphia - many of them students - who waited a half-hour to vote at 4 p.m.
More than 300 people had voted by then at the site, more than double that of a typical presidential primary.
"I have never seen a polling book this thick," said Sheryl Simons, a poll worker in the ward, whose voters include college professors, students and lower-income city residents. "The debates and the candidates coming to campus has really heightened interest."
Obama, an Illinois senator, has trailed Clinton, a New York senator, by a small margin in Pennsylvania polls in recent weeks.