Trains on Tokyo's crowded JR Yamanote Line were halted for more than five hours Monday, including the morning peak time, due to a loose overhead power cable, officials at East Japan Railway Co. said.

The problem, discovered at 8:15 a.m. at Tokyo Station, led to the cancellation or delay of more than 100 train runs, including some on the JR Keihin-Tohoku Line, which runs parallel to the Yamanote Line along part of its route, they said.

About 170,000 passengers were affected.

The cable came loose along a stretch of track between Yurakucho, Tokyo and Kanda stations, apparently because the 500 kg weights that were supposed to keep the cable taut became detached, JR East officials said.

The railway said it took time to restart operations because an iron bar used to stabilize the weights had also broken. The bar is about 2 cm in diameter.

The vital loop line was stopped for more than five hours until 1:17 p.m. Train runs on the outer loop and the Keihin-Tohoku Line were temporarily suspended by repair work on the inner loop.

The railway also checked train cars because their pantographs may have been damaged after operating in the area where the power cable was slack.

"The trouble should have been easily discovered by daily maintenance. We will ask JR to report further on the incident," an official of the Land, Infrastructure and Transport Ministry said.

Stations along the Yamanote Line were jammed with irritated commuters.

"This will severely affect my work. I must go to my office by using a bus or some other means," a 23-year-old office worker at JR Tokyo Station said.