A huge earthquake has killed at least 15,000 people in south-eastern Iran, government sources say.
The massive earthquake struck early on Friday
The quake had its epicentre near the ancient city of Bam - most of its buildings have been flattened.
Many people were believed to be buried under rubble. President Khatami has spoken of a "national tragedy" and urged all Iranians to help the victims.
A big relief operation is under way, with many foreign countries sending supplies and rescue workers to Iran.
The country suffers frequent earthquakes, with small tremors happening almost daily.
In one earthquake in 1990, 35,000 people died.
The quake in Bam - a city and surrounding area of more than 200,000 people - occurred at 0528 local time (0158 GMT).
It is thought many people were crushed as they slept.
There were scenes of intense grief in the city, with people weeping next to corpses shrouded in blankets.
"I have lost all my family. My parents, my grandmother and two sisters are under the rubble," Maryam, 17, told Reuters.
About 70% of the houses in Bam have been destroyed, Iranian state television reported.
Two of the city's hospitals have collapsed, crushing many staff, and remaining hospitals are full.
RECENT IRAN QUAKES
June 2002: more than 200 killed in the western Qasvin and Hamedan regions
May 1997: More than 1,500 killed in eastern Iran
February 1997: about 1,000 killed in north-western Iran
June 1990: 35,000 died in worst recorded disaster in Iran, affecting the Caspian regions of Gilan
and Zanjan. About half a million people made homeless
June 1981: More than 1,000 killed in quake that destroyed town of Golbaf
A huge relief operation involving ordinary Iranians, the army, Islamic volunteer groups and local rescue teams is under way.
Rescue teams are flying to the area by plane and helicopter.
Emergency centres set up in makeshift buildings are now trying to care for the tens of thousands who have been injured.
The BBC's Miranda Eeles in Tehran says that in many places there simply is not enough room.
Telephones, electricity and water supplies have been cut by the tremor.
The concern now is how to rescue those still alive but buried deep under the rubble, our correspondent adds.
Temperatures at night in this desert city drop to well below freezing.
A number of countries - including Russia, Britain, Germany and Spain - have offered to send relief teams and supplies.
The United States offered humanitarian assistance. "Our thoughts and prayers are with those who were injured and with the families of those who were killed," White House spokesman Scott McClellan said.
The United Nations said it was sending experts to co-ordinate the aid effort.
The Red Crescent is setting up tents to house survivors.
Friday's quake had a magnitude of at least 6.3, according to Iranian sources. The US Geological Survey measured it at 6.7.
Bam - about 1,000 kilometres (620 miles) south-east of Tehran - was on the Unesco's list of World Heritage Sites.
An important regional centre in the 16th and 17th centuries, it contained many ancient buildings that were not built to withstand earthquakes.
Its 2,000-year-old mud-brick citadel is reported to have been destroyed.
Since 1991, tremors have claimed some 17,600 lives and injured 53,000 people, according to official figures.
Reuters reports that there is little modern earthquake education in Iran.
"Most people think that what God wills will happen. This is absolutely wrong. This thinking is poisonous," Tehran University professor Bahram Akasheh told the news agency.