(Updates with earthquake, pollution, three injured)

By Anna Mudeva

SOFIA, July 3 (Reuters) - A series of powerful explosions rocked an arms disposal depot storing 1,500 tonnes of obsolete munitions outside Sofia on Thursday, injuring three people, shaking apartment blocks and panicking thousands.

The explosions followed a fire at the complex near the village of Chelopechene on the eastern edge of the Bulgarian capital, police and civil defence officials said.

Sofia was later shaken by a 3.2-magnitude earthquake, the geological institute said. It said there was no link to the blasts. The tremor's epicentre was about 15 km (10 miles) south of the capital and caused no casualties or damage.

The explosions continued for about eight hours. Authorities said smaller blasts were likely in the next 24 hours and urged Sofia's nearly two million citizens to stay calm.

Checks showed increased emissions of sulphur and nitrogen dioxide in the air in eastern and north-eastern suburbs but civil defence officials said human health was not threatened.

Sofia's international airport to the east of the capital was shut temporarily after debris from the blasts fell on the runway. Incoming flights were diverted to the southern city of Plovdiv.

"This morning at 6:30 a.m. (0330 GMT) a fire broke out in the area of Chelopechene and explosions followed," the emergencies ministry said in a statement.

Guards at the arms complex, which is attached to a military base that is being closed down, were evacuated.

Three people from nearby villages were injured, police said. Two suffered cuts from flying glass and the third was taken to hospital with breathing difficulties.

About 2,000 people from Chelopechene and nearby villages were evacuated, while others fled their homes in panic.

"We were scared to death," Nadka Doncheva, 61, who lives in the village, told Reuters. "All the windows are broken."

"The house was shaking. I thought it was an earthquake," a tearful woman from Chelopechene told national radio.

Authorities are investigating the reason for the explosions.

The blasts shook apartment blocks and broke windows in eastern districts of Sofia, witnesses said. Some people dashed into the streets in panic.

"The threat is over. The danger arising from the 20 tonnes of TNT stored there has passed because it had burnt out," Defence Minister Nikolai Tsonev said after an emergency cabinet meeting.

"The explosions are declining. There is no reason for people to fear dangerous air pollution," he added.

NATO member Bulgaria is due to destroy up to 100,000 tonnes of Soviet-era munitions, officials say.

Power utilities started restoring electricity supplies to industrial plants and villages near the site of the blasts. (Additional reporting by Tsvetelia Ilieva and Oleg Popov; Editing by Janet Lawrence) (anna.mudeva@reuters.com; +359 2 9399 733; Reuters Messaging: anna.mudeva.reuters.com@reuters.net))

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