People pray during mass celebrated outside Port-au-Prince's main cathedral on Sunday. (Gregory Bull/Associated Press)UN Secretary General Ban Ki-moon described the aftermath of last week's earthquake in Haiti as "one of the most serious crises in decades," as aid groups hurried to reach increasingly desperate survivors.

"The damage, destruction and loss of life are just overwhelming," he said before flying to Haiti on Sunday.

UN Secretary General Ban Ki-moon, left, walks with Acting Special Representative of the Secretary General Edmond Mulet in front of the damaged Haitian national palace in Port-au-Prince on Sunday. (Logan Abassi/Reuters)The UN secretary general said the World Food Program was already feeding 40,000 survivors and hopes to feed one million a day within a month.

"The challenge at this time is how to co-ordinate all of this outpouring of assistance," he said.

Ken Keen of the U.S. Southern Command has called the situation "a disaster of epic proportions with tremendous logistical challenges."

The World Health Organization said eight hospitals in Port-au-Prince were destroyed or damaged when the 7.0-magnitude quake hit on Jan. 12.

The largest hospital in the city, l'Hôpital Général, was functioning but was overwhelmed by fatalities and casualties on the weekend, according to World Vision. A number of donor countries have set up field hospitals to take the pressure off very limited medical resources.

About 180 tonnes of relief supplies arrived Saturday, but scores of people on the street say none of it is reaching them. Aid distribution continued to be slowed Sunday by bottlenecks at the capital's airport.

Some flights have been diverted to the neighbouring Dominican Republic, forcing affected aid groups to transport supplies into Haiti over narrow, damaged roads.

Security is also a logistical nightmare in Haiti. The country has no army and its police force has all but collapsed. About 2,000 international troops and police are maintaining law and order in Port-au-Prince. Some of a 1,000-strong Canadian Forces contingent expected to arrive early in the coming week are to assist in the task.

On Sunday, the bodies of two men could be seen on the street with their hands tied up after being shot by unidentified men who accused them of looting in the Delmas area of Port-au-Prince.

UN peacekeepers said anger is rising among the population and warned authorities and aid organizations to increase security to guard against looting.

Even with death and horror in the streets all around them, many Haitians gathered to pray at Sunday services wherever they could in the capital. One group attended a mass outside the city's main cathedral, which was destroyed in the quake.

Quake survivors gather Sunday near tents outside the presidential palace. (Angela Naus/CBC)

At a church service in Washington, D.C., U.S. President Barack Obama acknowledged the ongoing devastation and suffering in Haiti.

"Our Haitian brothers and sisters are in desperate need - bruised and battered," Obama told the congregation at the Vermont Avenue Baptist Church.

No one knows how many people died in the quake. Haiti's government alone has already recovered 20,000 bodies, not counting those recovered by independent agencies or relatives themselves, according to Prime Minister Jean-Max Bellerive. He said 100,000 deaths would "seem to be in the minimum."

Thousands of bodies are believed to be still buried beneath the rubble of collapsed buildings, and hundreds of thousands of Haitians are living in tents built with plastic sheets.

On a hillside golf course, perhaps 50,000 people were sleeping in a makeshift tent city overlooking the capital and paratroopers of the U.S. 82nd Airborne Division have set up a base for handing out water and food.

The slowness of the grim process of collecting bodies sparked a form of protest on both Saturday and Sunday. A group of men piled bodies to form a barricade on a primary road leaving Port-au-Prince.

The men responsible told the CBC the macabre protest worked the first day. UN trucks came to take away the bodies, so they set up the same kind of barricade on Sunday.

With files from The Associated Press