Rescue teams kept up search operations for dozens trapped beneath the rubble of collapsed houses in towns and villages in the mountainous area of the western province of Kermanshah that borders Iraq.
Iran's English-language Press TV said more than 450 people were killed and 7,000 were injured when the magnitude 7.3 earthquake jolted the country on Sunday. Local officials expected the death toll to climb as search and rescue teams reached remote areas of Iran.
The quake was felt in several provinces of Iran but the hardest hit province was Kermanshah. More than 300 of the victims were in Sarpol-e Zahab county in that province, about 15 km (10 miles) from the Iraq border.
Iranian state television said the quake had caused heavy damage in some villages where houses were made of earthen bricks. The quake also triggered landslides that hindered rescue efforts, officials told state television.
At least 14 provinces in Iran had been affected, Iranian media reported. Iranian media reported that a woman and her baby were pulled out alive from the rubble on Monday in Sarpol-e Zahab, the worst hit area with a population of 85,000.
Relief workers said while much aid had been pledged, there was an immediate need for blankets, children's clothes, medicine and large cans to store drinking water. TV aired footage of some people weeping next to corpses shrouded in blankets.
Iranian Supreme Leader Ayatollah Ali Khamenei offered his condolences on Monday, urging all government agencies to do all they could to help those affected. State TV appealed for blood donations. The government announced one day mourning on Tuesday.
Tempers frayed in the quake-hit area as the search went on for survivors amidst the twisted rubble of collapsed buildings. State TV aired footage of damaged buildings, vehicles under rubble and wounded people wrapped in blankets.
An Iraqi meteorology official put the quake's magnitude at 6.5, with the epicentre in Penjwin in Iraq's Sulaimaniyah province in the Kurdistan region, close to the main border crossing with Iran.
Kurdish health officials said at least six people were killed in Iraq and at least 68 injured, adding that in northern Iraq Kurdish districts seven were killed and 325 wounded.
Iraq's health and local officials said the worst-hit area was Darbandikham district, near the border with Iran, where at least 10 houses had collapsed and the district's only hospital was severely damaged.
"The situation there is very critical," Kurdish Health Minister Rekawt Hama Rasheed told Reuters.
The district's main hospital was damaged and had no power, Rasheed said, so the injured were taken to Sulaimaniyah for treatment. Homes and buildings had extensive structural damage, he said.
The quake was felt as far south as Baghdad, where many residents rushed from their houses and tall buildings when tremors shook the Iraqi capital.
"I was sitting with my kids having dinner and suddenly the building was just dancing in the air," said Majida Ameer. who ran out of her building with her three children.
"I thought at first that it was a huge bomb. But then I heard everyone around me screaming: 'Earthquake!'"
"I lost nine members of my family ... they were killed while they slept," a weeping middle-aged woman told state TV.
Local officials said hundreds of critically wounded people had been transferred to other provinces as Kermanshah's main hospital had been badly damaged.
Iran sits astride major fault lines and is prone to frequent tremors. A magnitude 6.6 quake on Dec. 26, 2003, devastated the historic city of Bam, 1,000 km southeast of Tehran, killing about 31,000 people.
Electricity and water was cut off in several Iranian and Iraqi cities, and fears of aftershocks sent thousands of people in both countries out onto the streets and parks in cold weather.
Across the area, rescue workers and special teams using sniffer dogs and heat sensors searched wreckage. Blocked roads made it hard for rescue workers to reach some remote villages.
Iranian authorities acknowledged the relief effort was still slow and patchy. More than 70,000 people needed emergency shelter, the head of Iranian Red Crescent said.
Hojjat Gharibian, one of hundreds of homeless Iranian survivors, was huddled against the cold with his family in Qasr-e Shirin.
"My two children were sleeping when the house started to collapse because of the quake. I took them and ran to the street. We spent hours in the street until aid workers moved us into a school building," Gharibian told Reuters by telephone.
Iran's police, Revolutionary Guards and Basij militia forces were sent to quake-hit areas overnight, state TV reported.
Aid workers handed out water to people, while bulldozers worked to clear the streets in damaged areas.
A local official said aftershocks slowed down erecting tents for homeless people but electricity had been restored in most of the quake-hit areas.
"The main problem is sheltering people at this cold weather. We need more tents," Qasr-e Shririn governor Faramarz Akbari told TV.
(Writing by Parisa Hafezi, reporting by Raya Jalabi and Ahmed Rasheed in Iraq, Bozorgmeh Sharafedin in Londn, Tuvan Gumrukcu and Irem Koca in Ankara, Dan Williams in Jerusalem, and Dubai newsroom; Editing by Peter Cooney, Bill Tarrant, Richard Balmforth)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)