Hundred of police and soldiers were deployed to contain looters in Tacloban, the devastated provincial capital of Leyte, while the United States announced it had responded to a Philippine government appeal and would send military help.
"Tacloban is totally destroyed. Some people are losing their minds from hunger or from losing their families," high school teacher Andrew Pomeda, 36, told AFP, as he warned of the increasing desperation of survivors.
"People are becoming violent. They are looting business establishments, the malls, just to find food, rice and milk ... I am afraid that in one week, people will be killing from hunger."
Authorities were struggling to even understand the sheer magnitude of the disaster, let alone react to it, with the regional police chief for Leyte saying 10,000 people were believed to have died in that province alone.
"We had a meeting last night with the governor and, based on the government's estimates, initially there are 10,000 casualties (dead)," Chief Superintendent Elmer Soria told reporters in Tacloban.
"About 70 to 80 percent of the houses and structures along the typhoon's path were destroyed."
On the neighbouring island of Samar, a local disaster chief said 300 people were killed in the small town of Basey.
He added another 2,000 were missing there and elsewhere on Samar, which was one of the first areas to be hit when Haiyan swept in from the Pacific Ocean with maximum sustained winds of 315 kilometres (195 miles) an hour.
Dozens more people were confirmed killed in other flattened towns and cities across a 600-kilometre stretch of islands through the central Philippines.
The Philippines endures a seemingly never-ending pattern of deadly typhoons, earthquakes, volcano eruptions and other natural disasters.
This is because it is located along a typhoon belt and the so-called Ring of Fire, a vast Pacific Ocean region where many of Earth's earthquakes and volcanic eruptions occur.
However, if the feared death toll of above 10,000 is correct, Haiyan would be the deadliest natural disaster ever recorded in the Philippines.
The previous deadliest disaster in the Philippines occurred in 1976, when a tsunami triggered by a magnitude 7.9 earthquake devastated the Moro Gulf on the southern Philippine island of Mindanao, killing between 5,000 and 8,000 people.
Haiyan's maximum sustained wind speeds made it the strongest typhoon in the world this year, and one of the most powerful ever recorded.
Witnesses in Tacloban recalled waves up to five metres (16 feet) high surging inland, while aerial photos showed entire neighbourhoods destroyed with trees and buildings flattened by storm surges that reached deep inland.
"The effects are very similar to what I have seen in a tsunami rather than a typhoon," the Philippine country director of the World Food Programme, Praveen Agrawal, who visited Tacloban, told AFP.
"All the trees are bent over, the bark has been stripped off, the houses have been damaged. In many cases they have collapsed."
President Benigno Aquino said while visiting Tacloban that looting had emerged as a major concern, after only 20 out of 390 of the city's police officers turned up for work following the typhoon.
"So we will send about 300 police and soldiers to take their place and bring back peace and order," he told reporters in Tacloban.
"Tonight, an armoured vehicle will arrive and our armed forces will display the strength of the state to put a stop to this looting."
As the scale of the disaster began to emerge, an international aid effort began to ratchet up.
In Washington, the Pentagon announced that US defence secretary Chuck Hagel had responded to a request from the Philippines for military aid and directed the US Pacific Command to deploy resources.
United Nations leader Ban Ki-moon also promised that UN humanitarian agencies would "respond rapidly to help people in need", while the European Commission said it would give three million euros ($4 million) to help in relief efforts.
Haiyan moved out of the Philippines and into the South China Sea on Saturday, from where it tracked towards Vietnam.
Although it weakened out at sea, more than 600,000 people were evacuated in Vietnam ahead of its expected landfall on Monday morning.
Chinese state news-agency Xinhua meanwhile reported six members of a cargo boat were missing off the southern island province of Hainan.
Over 600,000 evacuated in Vietnam
More than 600,000 people were evacuated as super typhoon Haiyan bore down on Vietnam, authorities said on Sunday, after the storm smashed through the Philippines leaving thousands feared dead and causing widespread devastation.
Residents of the Vietnamese capital Hanoi were braced for heavy rains and flooding, while tens of thousands of people in coastal areas were ordered to take shelter ahead of Haiyan's expected landfall on Monday morning.
"We have evacuated more than 174,000 households, which is equivalent to more than 600,000 people," an official report by Vietnam's flood and storm control department said.
The storm is now expected to strike on Monday morning after changing course, prompting further mass evacuations of some 52,000 people in northern provinces by the coast, the VNExpress news site reported.
"People must bring enough food and necessities for three days ... Those who do not move voluntarily will be forced," the VNExpress report said, adding all boats have been ordered back to shore.
The Red Cross said in a statement Haiyan's changed path meant the "the disaster area could be enlarged from nine provinces to as many as 15", stretching the country's resources.
"This is one of the challenges going ahead," Michael Annear, Red Cross country representative, told AFP, adding that heavy rains and flooding were likely to hit Hanoi.
Reports on social media indicated many of the capital's residents were rushing to markets to stock up on food and drinking water before the storm hit.
Many of the estimated 200,000 people evacuated in four south-central provinces initially thought to be in the storm's path have been allowed to go back to their homes, according to a report on the government's website.
Haiyan "is quickly moving north and northwest, travelling at a speed of up to 35 kilometres per hour", the country's weather bureau said in a statement.
The weather system — one of the most intense typhoons on record when it tore into the Philippines — has weakened over the South China Sea and is expected to hit as a category 1 storm, meteorologists added.
The typhoon's epicentre is expected to make landfall around 7am on Monday (0530 IST), with winds of about 74 kilometres an hour.
At least four people were reportedly killed while preparing to escape the typhoon, disaster officials said.