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.
The BBC's Karishma Vaswani, in Padang, said that the Red Cross planned to hold a meeting in the city on Saturday to co-ordinate