Some three million of the nine million people in Haiti are thought to be in need of aid following the devastating earthquake four days ago.
The UN has launched an appeal for $562m (£346m), which it hopes will fund a six-month relief effort to help the injured and those who have lost loved ones, homes and livelihoods.
Because of Haiti's status as the poorest country in the western hemisphere, and owing to past disasters including several hurricanes in 2008, many international charities already had teams on the ground working on aid and reconstruction.
These agencies have mobilised emergency teams in response to the quake.
Medical aid to help treat the many thousands injured has been a priority, with the International Committee of the Red Cross reporting that most hospitals in Port-au-Prince are full or degraded.
The scale of the damage and the fact that it struck the capital city - the centre of resources - was handicapping the relief effort, Medecins Sans Frontieres' Marc Dubois told the BBC.
MSF says it has treated and stabilised more than 1,500 people in the capital since Tuesday's earthquake.
But after all three of its medical facilities in the city were damaged in the earthquake, it has been doing limited work mostly in tented clinics.
"We've got a full inflatable surgical hospital sitting, basically, at the airport, waiting to get on-line and working again," Mr Dubois said.
"This is a race against time - people are injured and need immediate first aid and they need surgery, and we've just now been able to get that going.
"But things like broken bones, real fractures, trauma wounds to the head require a specialised service.
"And to watch your children slowly get infected; if things don't happen right now ... then people are going to die from infections, not from their injuries."
MSF has begun some emergency surgery at Choscal Hospital in Cite Soleil, a slum area where tin-roofed shacks survived better in the quake than the concrete structures in other parts of the city.
The "Doctors Without Borders" charity has also found another site that it hopes to have up and running as an operating block this weekend.
The ICRC and Save the Children, both of which have worked in Haiti for many years, have been distributing medical supplies to hospitals and clinics.
Difficulties getting aid in and out of the capital's overwhelmed airport prompted a Red Cross medical convoy delivery by road on Saturday from the Dominican Republic.
The convoy transported a 50-bed field hospital, surgical teams and an emergency telecoms unit, and the Red Cross says it is planning the delivery of two larger field hospitals.
An ICRC flight carrying 40 tonnes of medical supplies that had been due to arrive on Friday was diverted to the Dominican Republic, and is due to reach Port-au-Prince overland on Sunday.
Care International, whose work in Haiti dates back to the aftermath of Hurricane Hazel in 1954, has identified water and food provision as its top priority in the earthquake's aftermath.
It has shipped 600,000 water purification sachets to Haiti from Panama and plans distribution of 60,000 meals of high-energy biscuits.
Another immediate need is supply of body bags, according to project manager Joseph Francoeur Jean in the nearby city of Gonaives, who called the disaster "Haiti's darkest day".
The Haitian government says it has collected 50,000 bodies, although there are fears the death toll could rise to between 100,000 and 200,000.
The day after the earthquake, the ICRC activated a special website to enable people affected by the disaster to get in touch with loved ones.
Of almost 20,000 registered on the site by Saturday, only about 1,400 of those were people inside Haiti letting others know they were safe and well.
The ICRC's Simon Schorno, in Port-au-Prince, said there were about 40 makeshift camps covering every open public space in the city.
"It's utter chaos - there is destruction in every neighbourhood," he said.
"People are walking around, looking for food, for help. Many are wearing facemasks to protect themselves from the smell of decaying bodies. There are no tents, no plastic sheeting, no place to cook and no toilets."
Save the Children said it had been assessing health needs in the temporary camps set up in the worst-hit neighbourhoods.
On the charity's website, emergency team leader Annie Foster described "dazed, dehydrated parents walking the streets with their children, searching for clean water, food, and shelter".
She said the charity was establishing "'safe space' areas for children in these camps, and also beginning child tracing programs to reconnect children who were separated from their families during the emergency".
Save the Children's other work includes:
Eleven ICRC emergency experts have already flown from Geneva, Switzerland, and are on the ground in Port-au-Prince assessing humanitarian needs.
Another specialist team is due to fly out on Sunday, ahead of 14 Emergency Response Units (ERUs) from Red Cross Societies around the world due to arrive in the coming days.
These ERUs will offer medical care, water and sanitation services, as well as setting up two "base camps" to provide logistical and technical support to the relief effort.