MADRID/LISBON, April 28 (Reuters) - Spain and Portugal

were hit by a widespread power blackout on Monday that paralysed

public transport, caused large traffic jams and delayed airline

flights, and utility operators were scrambling to restore the

grid.

Authorities were unable to explain the cause of the

outage at least an hour after it occurred, though a possible

cyber attack had not been ruled out and investigations were

ongoing, officials said. A crisis committee was set up in Spain

to manage the situation, according to people familiar with the

situation.

The Spanish and Portuguese governments convened emergency

cabinet meetings after the outage, which also briefly affected a

part of France, which borders northeastern Spain.

Portugal's utility REN confirmed a cut in electricity across

the Iberian Peninsula that also affected part of France, while

Spanish grid operator Red Electrica said it was working with

regional energy companies to restore power.

"All plans for the phased restoration of energy supply are

being activated, in coordination with European energy producers

and operators," a REN spokesperson said.

"REN is in permanent contact with official entities, namely

the National Civil Protection Authority. At the same time, the

possible causes of this incident are being assessed."

Play at the Madrid Open tennis tournament was suspended,

forcing 15th seed Grigor Dimitrov and British opponent Jacob

Fearnley off the court as scoreboards went dark and overhead

cameras lost power.

Spanish radio stations said part of the Madrid underground

was being evacuated. There were traffic jams at Madrid city

centre as traffic lights stopped working, Cader Ser Radio

station reported.

Hundreds of people stood outside office buildings on

Madrid's streets and there was a heavy police presence around

key buildings, directing traffic as well as driving along

central atriums with lights, according to a Reuters witness.

One of four tower buildings in Madrid that houses the

British Embassy had been evacuated, the witness added.

Local radio reported people trapped in stalled metro cars

and elevators.

Portuguese police said traffic lights were affected across

the country, the metro was closed in Lisbon and Porto, and

trains were not running.

Lisbon's subway transport operator Metropolitano de Lisboa

said the subway was at a standstill with people still inside the

trains, according to Publico newspaper.

A source at Portugal's TAP Air said Lisbon airport was

running on back-up generators, while AENA, which manages 46

airports in Spain, reported flight delays around the country.

In France, grid operator RTE said there was a brief outage

but power had been restored. It was investigating the cause.