San Francisco officials who took light rail Monday morning to kick off a citywide Transit Week event were held up when trains broke down on two Muni lines.

What was supposed to be a celebration of mass transit became an indictment of the city’s unreliable light rail system.

“This just shows how we need reinvestment in Muni — because we got to a point where it was the first day of Transit Week, and the L Taraval didn’t work,” said Janice Li, a candidate to represent San Francisco on BART’s Board of directors.

Li left her home in the Outer Sunset at 8 a.m. to take the L Taraval metro train from 42nd Avenue to the event organized by the San Francisco Transit Riders advocacy group. She intended to meet up with district Supervisor Katy Tang and others at 22nd Avenue to complete the trip to City Hall.

But 15 minutes after Li boarded her train, it still hadn’t moved. A muffled voice came over the PA system, warning passengers that “it was going to be a while,” Li said. She got off the train and began walking, passing crowds of stranded commuters on the sidewalks.

“Every time I passed a line of people, I’d shout, ‘The L is broken!’” said Li, who regularly takes the 7 Haight/Noriega bus, but seldom rides the metro rail.

“I felt like the town crier,” she said.

The L lurched back to life at about 8:30 a.m., and Li managed to board a packed train at 24th Avenue. She met Tang at West Portal station and they took the K metro line — a detour that Tang described an hour later, during a news conference on the steps of City Hall.

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It was a typical Monday for Muni, and a vexing illustration of the system’s many problems.

Across town in the Castro District, state Sen. Scott Wiener and several others met at the train stop near 20th and Church streets, only to find that the J metro train was delayed by nearly an hour.

The group marched eight blocks to the 16th and Mission BART station and caught an inbound train to Civic Center.

“Fortunately, we were all able-bodied and had some time flexibility, so we were able to do it with smiles on our faces,” Wiener said. “But many people can’t walk those eight blocks, or can’t afford to be that late for work and appointments.”

Having ridden Muni nearly every day for 21 years, he’s well aware of its shortcomings. But that didn’t stop him from lambasting the agency at the news conference and over social media.

“I think this morning highlighted the reality that Muni has lost control of its light rail system,” he said. “Vehicles break down, and Muni isn’t able to fix them in a timely manner.”

Tom Temprano, a legislative aide to Supervisor Rafael Mandelman, rode along with Wiener. He said the delay made him empathize with constituents who call “almost daily” to complain about issues on the J line.

San Francisco Municipal Transportation Agency director Ed Reiskin attended the news conference but didn’t speak. He offered a wan smile as other officials detailed the morning travails.

Rachel Hyden, head of San Francisco Transit Riders, assured that the criticisms come “from a place of love.”

Later, Reiskin told The Chronicle that although Muni is steadily adding new trains to its fleet, “we still rely predominantly on older trains that are increasingly difficult to maintain. Breakdowns of older trains cause ripple effects throughout the system.”

He promised to be “proactive about our service management and response, and to find innovative ways to improve reliability.”

Agency spokesman Paul Rose said the J line backed up because a door broke on an inbound train at 8:12 a.m. Maintenance crews managed to clear the train six minutes later, but the delay was long enough to jam the whole route.

The L train broke down at 8:15 a.m. because of a mechanical failure in its propulsion system, Rose said. Mechanical glitches hamstring about 2 percent of the 30,000 one-way trips that Muni’s light-rail trains take each month.

The maintenance issues on Monday came as the Muni reels from a systemwide slowdown caused by this summer’s closure of the Twin Peaks Tunnel, which reopened last month. During the shutdown, Muni officials poached buses and drivers from popular lines and used them to drive shuttles around the tunnel — a patched-together service plan that caused delays for riders throughout the city and drew a scalding letter from Mayor London Breed.

In a statement released Monday morning, Breed hailed Transit Week as “an opportunity to celebrate what is great about our transportation system, and focus on solving the challenges it faces.”

Not every official had a dissatisfying Muni experience to start the annual weeklong festivities. Supervisor Aaron Peskin said he caught the 41 bus in North Beach at 8:35 a.m., seamlessly transferred to the 47 on Van Ness Avenue, and arrived at City Hall half an hour early.

Peskin said his experience shows that Muni has successfully improved bus service over the past decade — by upgrading its fleet and adding vehicles — even as its light-rail service disintegrates.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan