Riders showing up to catch buses home Tuesday were not happy, and they voiced their displeasure.

Commuters arriving at the Transbay Transit Center to catch buses home on Tuesday afternoon were stunned to find their gleaming, 6-week-old transportation base abruptly closed because of a potentially dangerous defect.

“This is par for the course for San Francisco,” said Eric Tisch, 39, of Oakland, who was redirected just after 5 p.m. to the grounds of the old Temporary Transbay Terminal two blocks away. “If it’s a crack in a steel beam, well, that’s generally not something they’ll fix overnight.”

The Temporary Transbay Terminal — which had been mothballed behind chain-link fencing last month — was brought back to life, fast, as an emergency backup. Arriving commuters looking for their buses there had to walk around the fencing to find their way in.

Once inside, they searched their phones for information as they scrambled to locate their buses. The signs and timetables that once guided them were taken down after the new Transbay Transit Center opened Aug. 12.

Some commuters had been inside the new transit center waiting on the third floor for their buses when they were told to leave. One of them was Lora Martinenko, who said she had been unaware that the cracked steel beam was looming just above her when officials began ushering riders down escalators.

“They hadn’t said anything about this until now,” Martinenko said.

Confused, the passengers hurried toward the old terminal, asking officials for more information only to be met by a curt, “That’s all I can tell you.”

Reaction on social media to the sudden closure of the $2.2 billion center was swift. “This building is doomed!” “The city is falling apart in more ways than one.”

“That whole area feels like a death trap,” wrote a woman named Nette, who said in her profile that she’s a San Francisco native.

Indeed, the new terminal sits in the shadow of San Francisco’s infamous Millennium Tower, the victim of another kind of crack — in a 36th-floor window — of mysterious, ominous origins. Plus there has been sinking and tilting that has made that building an unexpected tourist favorite.

The neighborhood does seem a bit cursed of late. Just a month after the Transbay Transit Center’s grand opening, the walkways on its rooftop garden started to crumble with potholes.

At the drab temporary terminal, frustration boiled over at times.

“I made the schlep all the way to the (new) terminal and back, then they made me walk around this fence,” said Jean Walsh, 49, who was trying to head home to Oakland on the F-bus.

Carin Sauerwein, 32, found out about the cracked beam as she waited to board the F-bus back home to Oakland.

“That’s horrifying,” she said. “I would not want to continue using the transit center.”

Sauerwein said she would consider taking her car rather than the bus.

“This makes me question the whole system,” she said.

Sauerwein stood in a line that snaked along the temporary steel fencing at the old Transbay Terminal with dozens of other riders eager to get home. Riders scrolled through Facebook and Twitter, looking for clues of the closure.

Emily Choi, 25, said she couldn’t believe what was happening.

“I am definitely kind of shocked,” Choi said. “It’s a brand new terminal, and now they have people standing in front of it saying it’s closed and we should go to the old one.”

Rene Dicherri, 36, said she believed that transit officials waited too long to inform riders as they shuffled to catch buses, instead of sending alerts ahead of time.

“This could have been messaged somehow,” Dicherri said. “Now I’m definitely going to miss my bus for sure.”

Two hours after the center was evacuated, commuters continued to show up at the new terminal, only to find entrances and sidewalks blocked by dozens of police officers and transit officials.

“Head to Howard and Beale to the temporary terminal,” officials barked at pedestrians shuffling along Howard, refusing to provide any additional information.

Pedestrians clung to police tape lining the sidewalk of Beale Street, trying to catch a glimpse of any movement inside.

One passerby joked to friends that the center may collapse at any moment as they shuffled past the lip of the center.

“That’s horrifying. I would not want to continue using the transit center.” Carin Sauerwein, 32, commuter from Oakland