Doctors and search dogs, troops and rescue teams flew to devastated Haiti, a land of dazed, dead and dying people, finding bottlenecks everywhere, beginning at a main airport short on jet fuel and ramp space and without a control tower.
The international Red Cross estimated 45,000 to 50,000 people were killed in Tuesday's cataclysmic earthquake, based on information from the Haitian Red Cross and government officials.
Hard-pressed recovery teams resorted to using bulldozers to transport loads of dead.
Worries mounted, meanwhile, about food and water for the survivors.
"People have been almost fighting for water," aid worker Fevil Dubien said as he distributed water from a truck in a northern Port-au-Prince neighbourhood.
A handful of rescue teams were able to get down to work, scouring the rubble for survivors.
In one "small miracle," searchers pulled a security guard alive from beneath the collapsed concrete floors of the UN peacekeeping headquarters, where many others were entombed.
But the silence of the dead otherwise was overwhelming in a city where uncounted bodies littered the streets in the 80-degree heat, and dust-caked arms and legs reached, frozen and lifeless, from the ruins.
Outside the General Hospital mortuary, hundreds of collected corpses blanketed the parking lot, as the grief-stricken searched among them for loved ones. Brazilian UN peacekeepers, key to city security, were trying to organise mass burials.
Patience already was wearing thin among the poorest who were waiting for aid, said David Wimhurst, spokesman for the UN peacekeeping mission.
"They want us to provide them with help, which is, of course, what we want to do," he said. But they see UN vehicles patrolling the streets to maintain calm, and not delivering aid, and "they're slowly getting more angry and impatient," he said.
In Washington, President Barack Obama announced "one of the largest relief efforts in our recent history," starting with 100 million US dollars in aid.
The US Southern Command reported the first 100 of a planned 900 paratroopers of the 82nd Airborne Division landed in Haiti from North Carolina to support disaster relief, to be followed this weekend by more than 2,000 Marines.
The American troops "will relieve pressure" on overworked UN elements, Mr Wimhurst said.
From Europe, Asia and the Americas, other governments, the UN and private aid groups were sending planeloads of high-energy biscuits and other food, tons of water, tents, blankets, water-purification gear, heavy equipment for removing debris, helicopters and other transport, and teams of hundreds of search-and-rescue, medical and other specialists.
But two days after much of this ramshackle city was shattered, the global helping hand was slowed by the poor roads, airport and seaport of a wretchedly poor nation.
Some 60 aid flights had arrived by midday, but they then had to contend with the chokepoint of an overloaded Toussaint L'Ouverture International Airport.
At midday, the Federal Aviation Administration said it was temporarily halting all civilian flights from the US at Haiti's request, because the airport was jammed and jet fuel was limited for return flights.
The control tower had been destroyed in Tuesday's tremor, complicating air traffic. Civilian relief flights were later allowed to resume.
"There's only so much concrete" for parking planes, US Air Force Colonel Buck Elton said at the airport. "It's a constant puzzle of trying to move aircraft in and out."
Teams that did land then had to navigate Haiti's inadequate roads, sometimes blocked by debris or by quake survivors looking for safe open areas as aftershocks still rumbled through the city.
The UN World Food Programme said the quake-damaged seaport made ship deliveries of aid impossible.
The looting of shops that broke out after the 7.0-magnitude quake struck late Tuesday afternoon added to concerns. The Brazilian military warned aid convoys to add security to guard against looting by the desperate population.
"There is no other way to get provisions," American Red Cross representative Matt Marek said of the store looting.
"Even if you have money, those resources are going to be exhausted in a few days."
The city's "ti-marchant," mostly women who sell food on the streets, were expected to run out soon. Red Cross officials have estimated one-third of Haiti's nine million people are in need of aid.
The quake brought down Port-au-Prince's gleaming white National Palace and other government buildings, disabling much of the national leadership. That vacuum was evident.
No senior Haitian government officials were visible at the airport, although President Leonel Fernandez of the neighbouring Dominican Republic said after meeting with President Rene Preval that the Haitian leader was in control of the situation, working from the airport.
"Donations are coming in to the airport here, but there is not yet a system to get it in," said Kate Conradt, a spokeswoman for the Save the Children aid group.
"It's necessary to create a structure to stock and distribute supplies," the Brazilian military said.
Edmond Mulet, a former UN peacekeeping chief in Haiti, arrived from UN headquarters in New York to lead the relief effort, along with a UN disaster coordination team.
The first US military units to arrive took on a coordinating role at the airport, but State Department spokesman PJ Crowley underlined: "We're not taking over Haiti."
Mr Wimhurst said the Haitian police "are not visible at all," no doubt because many had to deal with lost homes and family members, and law-and-order needs had fallen completely to the 9,000 UN peacekeepers and international police in Haiti.