There were no immediate reports of damage from the second quake.
Rescuers have been struggling in heavy rain to try to find survivors from the first quake who are buried under rubble in Padang.
A doctor working in the city told the BBC: "There's quite a few people that have died. At first I was thinking it was going to be in the hundreds but it's going to be in the thousands of people that have been crushed or trapped."
One resident of Padang, Siti, told Reuters news agency: "We need aid as soon as possible. We need food and medicine. Our houses have collapsed."
Earlier Priyadi Kardono, a spokesman for Indonesia's National Disaster Agency, said more than 500 houses and buildings had collapsed.
"Many people are staying outdoors and some people are staying in public facilities," he told Reuters.
Mr Kardono said about 150 military personnel, as well as police and health ministry workers, were in the affected area, but they urgently needed heavy machinery to lift the rubble.
The quake brought down telephone lines, severely affecting communications with the region and making it difficult to assess the scale of the damage.
Speaking on Wednesday, Indonesia's President Susilo Bambang Yudhoyono said a coordinated relief effort was underway.
He said military planes were flying medical equipment and personnel to the affected region.
Witnesses to the first quake said residents ran out of buildings in Padang - which has a population of 900,000 - and surrounding cities.
MAJOR INDONESIAN QUAKES
26 Dec 2004: Asian tsunami kills 170,000 in Indonesia alone
28 March 2005: About 1,300 killed after a magnitude 8.7 quake hits the coast of Sumatra
27 May 2006: Quake hits ancient city of Yogyakarta, killing 5,000
17 July 2006: A tsunami after a 7.7 magnitude quake in West Java province kills 550 people
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