* Officials say highly explosive materials stored in port
* Blast felt in Cyprus, more than 100 miles away
* Prime minister calls for day of mourning
* Blast rocks Lebanese port area: https://graphics.reuters.com/LEBANON-SECURITY/BLAST/xklvydjjqpg/chart.png (Adds prime minister calls for national day of mourning)
By Samia Nakhoul and Yara Abi Nader
BEIRUT, Aug 4 (Reuters) - A huge explosion near the centre of Beirut killed at least 10 people, injured hundreds and sent shockwaves across the Lebanese capital on Tuesday, shattering windows and causing apartment balconies to collapse.
The most powerful explosion to hit Beirut in years shook the ground, leaving some residents thinking an earthquake had struck. Dazed and weeping, some of them wounded, people walked through streets checking to see if relatives were hurt.
The blast occurred in the city's port area. Lebanon's interior minister said initial information indicated highly explosive material, seized years ago, that had been stored there had blown up. Lebanon-based broadcaster Mayadeen cited the country's customs director saying tonnes of nitrate exploded.
Footage of the explosion shared by residents on social media initially showed a column of smoke rising from the port district followed by an enormous blast that sent a ball of white smoke and fireball into the sky. Those filming the incident from high buildings in other areas of the city were thrown backwards by the shock.
At least 10 bodies were taken to hospitals, a security source and a medical source told Reuters. The Lebanese Red Cross said hundreds of people were taken to hospitals for treatment.
Lebanese President Michel Aoun called for an emergency meeting of the country's Supreme Defence Council, according to the presidency's Twitter account. Prime Minister Hassan Diab called for a day of mourning on Wednesday.
The explosion occurred three days before a U.N.-backed court is due to deliver a verdict in the trial of four suspects from the Shi'ite group Hezbollah over a 2005 bombing which killed former prime minister Rafik al-Hariri and 21 other people.
It was not immediately clear what caused the blaze that set off the blast.
SCREAMING AND RUNNING
Internal Security Chief Abbas Ibrahim, touring the port area, said he would not pre-empt investigations. An Israeli official said Israel, which has fought several wars with Lebanon, had nothing to do with the blast.
The governor of Beirut port told Sky News that a team of firefighters at the scene had "disappeared" after the explosion.
"I saw a fireball and smoke billowing over Beirut. People were screaming and running, bleeding. Balconies were blown off buildings. Glass in high-rise buildings shattered and fell to the street," said a Reuters witness.
Residents said glass was broken in houses from Raouche, on the Mediterranean city's western tip, to Rabieh 10 km (6 miles) east). For a long time after the blast, ambulance sirens sounded across the city and helicopters hovered above.
The health minister told Reuters there was a "very high number" of injured. Al Mayadeen TV said hundreds were wounded.
Another Reuters witness said she saw heavy grey smoke near the port area and then heard an explosion and saw flames and black smoke: "All the downtown area windows are smashed and there are wounded people walking around. It is total chaos."
U.N. spokesman Farhan Haq told reporters it was not immediately clear what the cause was, and that there was no indication of any injuries to any U.N. personnel.
"We do not have information about what has happened precisely, what has caused this, whether it's accidental or manmade act," he said.
In Cyprus, a Mediterranean island lying 110 miles (180 km) northwest of Beirut, residents reported hearing two large bangs in quick succession. One resident of the capital Nicosia said his house shook, rattling shutters.
(Reporting by Samia Nakhoul, Yara Abi Nader and Laila Bassam; Additional reporting by Dubai and Beirut bureau; Writing by Lisa Barrington, Ghaida Ghantous and Dominic Evans; Editing by Gareth Jones and Edmund Blair)
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