It was described as the biggest earthquake to hit Iran in half a century, a 7.8 magnitude trembler that swayed buildings over an area stretching from Abu Dhabi to New Delhi.
According to early but conflicting reports, there were surprisingly few casualties. Iranian state television said at least five people had been injured, denying earlier reports that 46 people had been killed.
But an Iranian crisis-centre official, Morteza Akbarpour, said: "The epicentre of the quake was located in the desert, and population centres do not surround it. There were no fatalities in the towns around the epicentre."
Pakistan seemed to have borne the brunt of the shock. At least 34 people were reported killed there while several hundred mud-brick houses collapsed.
The US Geological Survey put the preliminary magnitude at 7.8 and at a depth of 82 kilometres, while Iran's seismological centre reported magnitude as 7.5 and centred near Saravan, a sparsely populated rural area about 50km from the Pakistani border.
An Iranian Red Crescent official, Morteza Moradipour, said emergency crews, including dog teams to sniff through the debris for any buried survivors, had reached the area.
"Because of the strength of the earthquake we had expected to see significant damage in residential areas but the quake was at a depth of 95km and therefore the extent of the damage was on a par with earthquakes measuring magnitude 4," he said.
Ten years ago, 30,000 Iranians were killed in a far smaller 6.6-magnitude quake that hit the south-east city of Bam. But the impact of any quake depends on its depth, the geological formation of the area it hits, and the population density of the stricken area.
Experts said the depth of yesterday's quake was the reason for the low level of damage.
The trembler struck less than a week after a 6.1-magnitude quake hit close to Iran's only nuclear power station at Bushehr, killing 37 people and injuring 850.
Russia, which helped to build the Bushehr plant, again sought to reassure Iran's Gulf Arab neighbours that the reactor had not been damaged in yesterday's powerful quake.
But the tremors were "ominous reminders of how earthquake-prone Iran's terrain truly is and how critical it is for the Iranian government to be prepared for a nuclear emergency", said Ali Vaez, an Iran expert at the International Crisis Group.
High-rise offices were evacuated in the UAE, Bahrain, Oman and Qatar, as authorities called for calm.
People also fled some buildings in New Delhi. Salim Khan, who runs a stall selling grilled meat in the Indian capital, described people running in panic, screaming, "bhookamp", or earthquake in Hindi.
"It is common to feel tremors here from time to time so we did what we always do," Mr Khan, 24, said. "We pulled down our shutters and everyone gathered outside their shops."
The quake also shook large parts of Pakistan, including Islamabad. It was not, however, felt in the Iranian capital, Tehran, a sprawling city of 12 million.
* With additional reporting from Surya Bhattacharya in New Delhi and Elizabeth Dickinson in Abu Dhabi
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