The quake left more than 45,000 homes destroyed or damaged and major roads blocked.
UN officials believe that more than double the authorities' figure of 50,000 people have lost their homes.
As well as the urgent need for emergency shelter, relief agencies say that about 15,000 people are in urgent need of food aid, and as many as three million people are without water supplies.
The 22-year-old musician - whose rescue was broadcast around the world - died of heart failure hours after an emergency operation to amputate his leg.
The BBC correspondent in El Salvador says the 40% of the country's population who live in poverty are likely to be hardest hit as the quake has destroyed their marginal livelihoods.
More than 675 people are known to have died, while hundreds of others are missing.
The dead are being buried as quickly as possible, often in mass graves, to prevent the spread of disease.
Serious damage
The UN believes the damage is far worse than that caused by Hurricane Mitch two years ago, and that reconstruction efforts will take months.
The quake left more than 45,000 homes destroyed or damaged and major roads blocked.
About 20,000 people have been evacuated from areas at risk from further landslides caused by the continual aftershocks.
With little prospect of finding any more people alive in the rubble of collapsed buildings, the authorities have switched their attention from rescue to relief.
International aid is flowing into the country, while the Salvadorean army has been mobilised to distribute food, tents and blankets to the homeless.
Helicopters vital
Aid agencies are relying on helicopters to distribute much of the aid as landslides have rendered many of the country's roads impassable.
"All we have is God and the helicopter that brings food," said housewife Fidelia Guardao.
President Francisco Flores has thanked the international community for its help in mounting a massive relief operation.
Saturday's earthquake also hit Guatemala, Honduras, Mexico, Costa Rica and Nicaragua.
The last major earthquake in El Salvador was in 1986 when 1,400 people died.