More than 1,000 people are known to have died and up to 3,000 more are believed trapped beneath collapsed buildings in the city of Padang.
Widespread destruction has also been reported in surrounding areas.
Australia, Russia, the UK and the EU are among those sending emergency supplies, medics and rescue teams.

Witnesses in Padang report a stench of decomposing bodies hanging over collapsed buildings as rescuers battle to reach survivors.
Food shortages are being reported in areas where the earthquake has damaged roads, while people who lost their homes are said to be sleeping in the streets.

Despite a lack of heavy lifting equipment, rescuers are still pulling survivors from the rubble.

Ratna Kurnia Sari, 19, was rescued badly injured but alive from the rubble of her college in Padang 40 hours after the earthquake struck.

Hundreds of buildings in Padang collapsed when the quake struck
“Her dead friends were beneath and above her,” said Dr Dubel Mereyenes, who was treating her.

But the BBC’s Alastair Leithead, who is in Padang, says that as the hours pass, the rescue operation is turning into the recovery of bodies.
Two Australian planes carrying medical personnel and rescue experts have arrived in Padang, and dozens of British firefighters are due there late on Saturday.

A Swiss sniffer-dog team is already on the ground, and Russian, Estonian and Japanese personnel have all been sent. Countries around the world have pledged relief funds.

Indonesian President Susilo Bambang Yudhoyono has also called for $10m (£6.2m) in government aid to be distributed quickly.
“The… fund has to flow quickly, no more bureaucracy for this,” he said. “This is an emergency, so speed is crucial.”

The Red Cross in Geneva said aerial photos suggested the disaster zone was wider than previously thought, stretching across western Sumatra.
“The feedback is that Padang city and environs are bad, but once you go into the surrounding rural areas, the situation is very grave,” said spokesman Christine South.

“There was talk of complete devastation of some villages – 100% devastation – and 50% in others.”
Wednesday’s 7.6 magnitude quake struck off the coast of Padang and caused devastation. A second quake of 6.8 struck nearby on Thursday causing panic but no reports of damage or casualties.

Source: BBC