Independence Visitor Center was evacuated for about an hour Sunday afternoon while police searched the tourist attraction and adjacent garage for a bomb or bombs.
Visitors and employees were rushed out of the complex at Sixth and Market Streets shortly after noon and allowed back in around 1:30 p.m.
In the interim, the Police Department bomb squad searched the facility with dogs, but found no explosives.
"We took necessary precautions," Lt. Joseph McGarrey said.
The center, across Market Street from the Liberty Bell, was evacuated after a man in camouflage fatigues who police labeled suspicious was found in a garage, sitting in a parked vehicle loaded with paper, clothing, and other items, McGarrey said. The man, whose identity was not released, was detained for questioning, but no charges had been filed as of Sunday afternoon, McGarrey said. He suggested the man might have mental issues.
The heavy law enforcement response in one of Philadelphia's most popular tourist areas demonstrated the heightened level of security in the city since last Monday's twin bombings near the Boston Marathon finish line that killed three people and wounded more than 180.
The terror attack led to a manhunt that shut down Boston and its suburbs Friday and ended with the death of one of the suspected bombers and the arrest several hours later of his 19-year-old brother, who remains hospitalized in serious condition.
In Philadelphia on Sunday, Temple University student Ashton Seiwell, 21, who was among those evacuated from the Independence Visitor Center, asked: "What's going on in the world?"
Seiwell, who works for a vendor at the center, Wheel Fun Rentals, said police ordered him and all tourists and employees outside around 12:15 p.m.
"I'm not really that nervous," he said as he waited to return to work. "I'm not too worried. I'm more intrigued. I just want to know what's happening."
Seiwell said the law enforcement response made him feel secure. The center had been blocked with police tape along the Sixth Street side.
The investigation was a joint effort by the National Park Service, Philadelphia police, and transit police.
After authorities gave the all-clear, the center was reopened and tourists began entering the building again in large numbers.