By Jan Lopatka and Vera Dvorakova

PRAGUE (Reuters) -A power outage in large parts of the Czech Republic, including Prague, trapped people in public transport and lifts and idled factories on Friday after a fallen high-voltage cable disrupted the network.

The incident is likely to add to concerns about the resilience of Europe's power infrastructure after Spain suffered the worst blackout in its history in April and a fire knocked out the power supply to London's Heathrow airport in March.

"There was a massive power outage in part of Prague and in the northern and eastern Czech Republic around 12 pm (1000 GMT) today," the Industry and Trade Ministry said. "The cause was the fall of a power cable, not a cyberattack nor a failure of renewable resources."

The outage was nearly fully resolved by 1600 GMT.

It halted international and local trains and public transport in several cities, including Prague, where the underground was briefly shut down and trams were at a halt for hours. About 1,000 mobile phone network stations were affected and ran on back-up systems.

National transmission system operator CEPS declared a nationwide state of emergency after the V411 transmission grid line and the Unit 6 of the Ledvice power plant failed. It was not known what caused the power cable to fall.

This had knock-on effects, overburdening another line and substation, and forcing part of the grid to operate as an island, cut off from other parts of the European grid.

The Czech Republic has dozens of substations - facilities that convert electricity into different voltages so it can be transmitted throughout the country and distributed locally.

CEPS had earlier said the fallen line on the 45 kilometre (29 miles) high-voltage line in the northwest of the country had affected eight of these substations and caused blackouts in five of the Czech Republic's 14 regions.

The fallen line, serving an area with lignite power plants, has been included in a modernisation plan and is due to be doubled in capacity by 2028.

The Ledvice 6 power plant, which was also affected, is a 660 megawatt, coal-fired plant built in 2017 and operated by CEZ. CEZ did not comment on the plant.

SYSTEM RESTORED

All affected substations had power back before 1300 GMT, CEPS said, but distribution companies were working for hours more to restore supplies to customers.

Prime Minister Petr Fiala told a briefing that about 2,000 customers remained without power just before 1600 GMT. He said around half a million had been affected earlier.

Across the country, the outage caused 215 incidents involving people trapped in elevators, fire brigade spokesperson Lucie Pipis told Reuters, adding everyone had been rescued.

The prison authority said 13 prisons had lost power, but that security had not been compromised. Three large hospitals in Prague temporarily ran on back-up power.

Orlen Unipetrol's Czech refinery and chemical plant at Litvinov went into emergency shutdown, the company said on X.

After power supplies resumed, it began restarting operations, but said the process would take several days.

Following the outage in Spain in May, analysts said that Europe's ageing power grid and lack of energy storage capacity will require trillions of dollars in investments to cope with rising green energy output and increasing electricity demand.

(Reporting by Jan Lopatka, Vera Dvorakova, Nina Chestney, writing by Alan Charlish; Editing by Mark Potter and Barbara Lewis)