PADANG ? (UPDATE) Indonesia said Thursday it feared thousands had died in an enormous earthquake as exhausted rescue workers clawed through mountains of rubble with their bare hands in a race to find survivors.

The first flights laden with food, medicine and body bags began arriving in the devastated region on Sumatra Island as another powerful quake struck further south, causing more injuries and sparking panic.

Wednesday afternoon's 7.6-magnitude quake toppled buildings and led to fires in Padang city, home to nearly a million people on the coast of Sumatra, which was left largely without power and communications.

The official death toll hit 529 with 105 seriously injured, but those numbers are expected to soar as the full scale of the tragedy unfolds. Many districts remain closed to emergency services.

"Our prediction is that thousands have died," health ministry crisis center head Rustam Pakaya said.

Rescue teams from the Indonesian army and health ministry descended on the city and surrounding towns to hunt for survivors in the twisted wreckage of collapsed buildings and homes, with work expected to go on into the night.

In pouring rain that hampered rescue work early in the day, overwhelmed police and soldiers clawed through the tangled remains of schools, hotels and the city's main M. Djamil hospital.

Padang, which lies between the Indian Ocean and the Bukit Barisan mountains, was a chaotic scene of traffic jams and rubble set against the constant din of sirens as ambulances tried to negotiate the gridlock.

At the M. Djamil hospital, injured residents were dropped off at hastily erected tents where doctors worked frantically to treat victims.

Emilzon, a medic who gave only one name, said they were treating hundreds of people for broken bones, head injuries and trauma, many of whom were car crash victims.

"We are running out of doctors and nurses because we are overwhelmed with patients," he said.

In front of a collapsed school, 49-year-old mother Andriana waited in tears as police picked through the rubble for her 14-year-old daughter and dozens of other children believed trapped inside.

"I've been waiting here since yesterday. I haven't been home yet and keep praying to God my daughter is alive," she said, her reddened eyes darting back and forth across the rubble.

Police said the bodies of eight children had been hauled from the school, a college where they had been taking extra classes, while another nine youngsters had been pulled out alive.

Authorities said they were suffering from a desperate shortage of heavy machinery but the military said planes loaded with tents and blankets had been dispatched to help the thousands left homeless by the disaster.

Indonesian President Susilo Bambang Yudhoyono, who returned from the United States and flew to Padang, said emergency services should prepare for the worst.

"It's better to overestimate than to underestimate," he told reporters. "It is better to send more enforcement, especially in emergency aid... which can help those who are still buried in the rubble."

The government said it had approved 26 million dollars in cash to help victims of the quake and international aid groups dispatched relief teams to the affected zone.

The quake struck on Wednesday off Sumatra's west coast, 47 kilometers (29 miles) northwest of Padang on a major faultline that scientists have long warned was set to release pent up energy.

A series of earthquakes in recent years, including one of magnitude 8.2 that struck Bengkulu province in September 2007 and another off Aceh that triggered the Indian Ocean tsunami of December 2004, have released energy along the faultline.

"West Sumatra is like a supermarket for geological disasters. There are active volcanoes, landslides, land quakes caused by faults," Geological Disaster Mitigation and Volcanology Centre head Surono told Agence France-Presse by phone.

Dozens of aftershocks followed Wednesday's quake, including a major one Thursday which the US Geological Survey measured at 6.8 and said struck on land 225 kilometers (140 miles) southeast of Padang.

The social affairs ministry said 12 people were seriously injured and 600 houses had collapsed in Jambi provincial district of Kerinci.

The Indonesian earthquakes followed a massive 8.0-magnitude tremor that spawned a deadly tsunami in the Samoan islands of the South Pacific.

Both disaster zones sit on the volatile "Ring of Fire," an arc of seismic instability around the Pacific Rim.