Tourists and workers gather outside the West Front of the Capitol following Tuesday’s earthquake. (Felicia Sonmez/Washington Post)

The earthquake that shook Washington Tuesday afternoon forced Capitol Hill staffers and other workers out of their offices and onto the streets – and led the Senate to conduct a rare session off the Capitol grounds.

Sgt. Kimberly Schneider, a spokeswoman for the U.S. Capitol Police, said in a memo to Hill offices late Tuesday afternoon that no serious earthquake-related injuries had been reported. Several of the buildings of the Capitol complex — which had been evacuated shortly after 2 p.m. — remained closed as of early evening, although the Capitol building itself had been inspected by the Architect of the Capitol’s office and was “ready for re-entry,” Schneider said.

“Persons with offices in the Capitol Building will be permitted entry to retrieve personal items and to secure their workspaces,” Schneider said in the memo. “Building inspectors are continuing to work and your cooperation is requested to limit time in the building to an absolute minimum.”

”It is anticipated that tomorrow will be a regular business day,” she added.

Most members of Congress were back in their home districts for the August recess and were not present at the Capitol.

One member who was present, however, was Sen. Chris Coons (D-Del.), who had been scheduled to preside over a brief “pro forma” Senate session, during which no legislative business is conducted. Both chambers have been holding such sessions throughout the August break, a move that effectively blocks President Obama from making any recess appointments while Congress is out of town.

Coons gaveled the Senate into a pro forma session that began just after 3:30 p.m. and lasted 22 seconds, according to a pool report of the session by Roll Call’s Jessica Brady. The session took place in a conference room in the basement of the Postal Square building next to Union Station, with 50 or 60 Capitol Police officers, Senate floor staffers and others present in the improvised chamber. As in most pro forma sessions, there was no prayer and no pledge of allegiance, according to the pool report.

Several blocks of Pennsylvania Avenue SE were temporarily closed to traffic Tuesday afternoon in the wake of the earthquake. Outside the West Front of the Capitol, about 60 Capitol workers and tourists were gathered just after the building was evacuated.

“There was a lot of noise and shaking — the floor, the walls, everything was moving,” said Robert Hawe, 54, who works in the electrical division at the Capitol and was in the basement when the quake happened. The shaking went on for about 10 or 15 seconds, he said; then the alarms at the Capitol went off and the building was evacuated.

“It sounded like an explosion,” said Kip, a decorative painter at the Capitol who was also in the basement when the quake struck. “The whole place shook. For that place to shake, it’s big.”

Scott Williams, a 37-year-old 911 dispatcher from Tacoma, Wash., who is visiting the District this week, said that he and his family were standing on a short wall just outside the West Front when they felt the wall beneath them move. They were planning to visit the Capitol and then go to a Nationals game later Tuesday night, but said they weren’t even sure now whether the Metro was running.

“Now we just have to figure out how to get home,” Williams said.

On D Street SE, just south of the House office buildings, traffic was moving and the sidewalks were bustling with helmet-clad construction workers, red-vested tour guides, BlackBerry-wielding staffers and other Capitol denizens waiting in the late-afternoon sun as the alarms from inside the building continued to blare.

The atmosphere on the street was festive; staffers and workers joked and chatted, and with no word on whether the Capitol might reopen later Tuesday, some headed for an early happy hour at several Capitol Hill cafes and bars.

Coons, who was briefed at about 2:55 p.m. regarding the offsite session by Secretary of the Senate Nancy Erickson, later told reporters at an impromptu news conference at the Upper Senate Park that he had arrived in Washington from Delaware by train earlier Tuesday and at first was concerned that the rumbling was something “far more serious” than an earthquake.

He found out from an interviewer who was questioning him over the phone about the conflict in Libya that the incident was in fact an earthquake. By mutual agreement by the Senate majority and minority leaders, he said, the pro forma session was held offsite.

“I was impressed by how rapidly and professionally the Capitol Police responded,” he told reporters.

Coons, who was presiding over his first pro forma session, said he had tried to contact the Senate Historian’s Office Tuesday to inquire about other instances during which the chamber had held sessions offsite — but, incidentally, the historian happened to be in Coons’s home state of Delaware Tuesday afternoon interviewing the senator’s predecessor, former senator Ted Kaufman (D).

The House had held a pro forma session before Tuesday’s quake, from 10 until 10:04 a.m., according to House Majority Leader Eric Cantor’s (R-Va.) office.

Another member of Congress who was present at the Capitol on Tuesday was Rep. Chris Van Hollen (D-Md.).

C.R. Wooters, Van Hollen’s chief of staff, said that staffers in the Maryland Democrat’s office on the top floor of the Longworth House Office Building thought the rumbling was coming from the office air-conditioning unit.

Then plaques and pictures started falling off the walls and bookshelves of the seventh-floor office, and staffers began to realize it was something more dire — potentially worse than an earthquake.

“I was around here for 9/11,” Wooters said. “So, you always get the 9/11 flashback.”

Van Hollen was in the office when the quake happened, Wooters added.

“He ushered everybody out, then went back in to make sure that the interns got out,” he said.