An official with the UN children's agency UNICEF in Geneva said at least 82 people were believed dead, but that figure could not immediately be confirmed.
"The number of people injured has become 257," Rosmini Savitri, a local government official in the disaster zone, told AFP news agency by phone.
The tremor hit on Tuesday in a mountainous area about 50km northeast of Padang, a city on Sumatra's western coast, recording a depth of 33km, the US Geological Survey said.
Hospitals were quickly overwhelmed and local officials appealed for urgent help.
Susilo Bambang Yudhoyono, Indonesia's president, had ordered police, military, ministers and authorities to co-ordinate the relief effort, a spokesman said.
"There is no electricity in [Solok] and it’s very hard for the authorities to get there as the road was blocked by landslides. There are only two small hospitals in Solok"
Step Vaessen, Al Jazeera's correspondent in Padang, Indonesia
Step Vaessen, Al Jazeera's correspondent in Padang, said there was considerable damage in Solok, a town two hours away.
"There are reports that hundreds of buildings collapsed there, mostly houses but also shops and banks," Vaessen said.
"There is no electricity in [Solok] and it’s very hard for the authorities to get there as the road was blocked by landslides. There are only two small hospitals in Solok," she said.
Vaessen said that efforts to treat people hurt in the quake were being frustrated by a lack of medical staff and supplies in Solok.
"There are not enough doctors or medicines, so everything has to come from Padang. The road has only just re-opened from Padang to Solok so aid is on its way now," she said.
The earthquake appeared to have been followed by at least one aftershock almost as strong as the first quake.
Many people were trapped in collapsed buildings and there was no official information about the situation at the quake's epicentre because phone lines were down, said Utjin Sudiana, West Sumatra's police chief.
"The epicentre is in Batusangkar but communication is disconnected from there so we don't know what the damage is," he added.
"Several houses have collapsed. There are hundreds of victims," Solok mayor Samsurahim told ElShinta radio, adding that a school had been burnt to the ground after the quake.
"We have asked for medical help," he said. "Our facilities here are insufficient."
Doctors were forced to set up outdoors, running drips for the injured and working in hastily erected tents.
Local television showed staff at the main hospital in nearby Padang scrambling to cope with the wounded.
The Indonesian archipelago sits on the Pacific Ring of Fire where continental plates meet, causing frequent seismic and volcanic activity.