A powerful earthquake hit off western Indonesia late Monday, briefly triggering a tsunami warning that sent thousands of panicked residents fleeing to high ground. There were no immediate reports of damage or injuries.

The 7.7-magnitude temblor struck at a depth of 13 miles (20 kilometers) off Sumatra island, said the U.S. Geological Survey.

At least five towns in the provinces of Bengkulu and West Sumatra were badly jolted, officials and witnesses said, as were the nearby Mentawai islands.

"Everyone was running out of their houses," said Sofyan Alawi, a resident in the city of Padang, adding that, with loudspeakers from mosques blaring out tsunami warnings, the roads leading to surrounding hills were quickly jammed with cars and motorcycles.

"We kept looking back to see if a wave was coming," said 28-year-old resident Ade Syahputra.

Areas closest to the epicenter of the 9:42 p.m. (10:42 a.m. EDT, 1442 GMT) quake were sparsely populated, and there were no immediate reports of damage or casualties, said Ade Edward, a disaster management agency official.

A 5.0-magnitude aftershock hit less than an hour after the original quake, and the region remained on alert for more.

Indonesia, the world's largest archipelago, is prone to earthquakes and volcanic activity due to its location on the so-called Pacific Ring of Fire.

The city of Padang was badly shaken one year ago by a 7.6-magnitude quake that killed at least 700 people and flattened or severely damaged 180,000 buildings.

That followed the 2004 tsunami off Sumatra's westernmost province of Aceh that was triggered by a 9.1-magnitude quake and killed 230,000 in a dozen countries, roughly half in Indonesia.