Rescue and relief teams from the army and health ministry, along with local residents, worked in pouring rain to hunt for survivors in the twisted wreckage of collapsed schools, hotels, shopping malls and other buildings.
The city's main M Djamil hospital was also damaged and tents were erected nearby where medical staff worked frantically to treat the constant stream of injured who had to be navigated through the city's chaotic traffic jams and rubble-strewn streets to get there.
Emilzon, a medic, said hundreds of people had been treated for broken bones, head injuries and trauma, mostly sustained when the quake hit.
"We are running out of doctors and nurses because we are overwhelmed with patients," he said.
Al Jazeera's Step Vaessen, reporting from Padang, said medical teams and military aircraft had been arriving with field hospitals, tents, medicine and food rations, but there did not appear to be enough equipment to conduct rescue operations.
The authorities admitted there was a desperate shortage of heavy machinery as police and soldiers clawed through the tangled remains of buildings.
Relatives of the missing gathered outside ruined buildings, but rescuers mostly found bodies, and only occasionally pulled out survivors.
Fauzi Bahar, Padang's mayor, appealed for help on Indonesian radio saying the city of 900,000 was "overwhelmed".
"We really need help. We call on people to come to Padang to evacuate bodies and help the injured," he said.
Susilo Bambang Yudhoyono, Indonesia's president, instructed officials to "flood" Padang with aid and medical relief.
Yudhoyono, who returned on Thursday from the US where he attended the UN General Assembly, flew to Padang to oversee the relief effort and to ask people to pray for a miracle to help the city.
He told emergency services to be prepared for the worst, saying it was "better to overestimate than to underestimate".
"It is better to send more enforcement, especially in emergency aid... which can help those who are still buried in the rubble."
Sri Mulyani, the finance minister, said the government had allocated $25m for a two-month emergency response.
She said the region and its infrastructure had been "damaged seriously" and warned that this would badly affect Indonesia's economic growth, as West Sumatra is a main producer of crude palm oil.
The 7.6 magnitude undersea earthquake struck on Wednesday evening, about 50km from Padang, flattening hundreds of buildings.
The tremor was felt in the Indonesian capital Jakarta, 940km away, and sent frightened office workers streaming out of buildings in Singapore as well as Malaysia's capital, Kuala Lumpur.
|Rescuers continue to hunt for survivors trapped under rubble [Reuters]
Dozens of aftershocks followed, as did a 6.8 magnitude earthquake that struck on Thursday morning, about 225km southeast of Padang, causing widespread panic and badly damaging houses but causing no casualties in Jambi, another Sumatran town.
Padang, the capital of Indonesia's West Sumatra province, sits on one of the world's most active fault lines along the so-called Ring of Fire, the same one that cracked off Aceh, at the northern tip of Sumatra, in 2004 to trigger the Indian Ocean tsunami.
That disaster killed more than 220,000 people in Indonesia, Thailand, Sri Lanka and India among other countries.
Padang was badly hit by an 8.4 magnitude quake in September 2007, when dozens of people died and several large buildings collapsed.
Scientists had been warning of a major earthquake in the area for a long time, but the Indonesian government has said it did not have funds for disaster-preparation measures, our correspondent said.
Geologists warn the low-lying city and surrounding area could be vulnerable to more seismic activity.
"There are three big volcanoes in West Sumatra - Merapi, Talang and Tandikat," Surono, the head of the Geological Disaster Mitigation and Volcanology Centre in Indonesia, said.
"We fear that this quake might cause volcanic eruptions there."