Cyber-attackers hacked into the European Commission on Wednesday, hours from a sensitive summit of EU leaders debating the military campaign in Libya, the euro debt crisis and nuclear safety, officials said.

"All staff were warned this morning" that "remote access to emails was no longer operational," a spokesman for the commission told AFP.

Pages from European Union websites, notably those from the European External Action Service, which handles diplomatic relations for the 27 EU states around the world, were also closed down, the spokesman said.

The attack was not isolated, in fact being a "fairly frequent" event, although the scale of this attack way outstripped previous ones, he underlined.

Up to 20,000 protesters are expected to converge on the summit that opens Thursday, but police said that initial authorisation that extended to union demonstrators being allowed to erect anti-austerity banners around EU headquarters on Wednesday had been partially withdrawn.

"The perimeters have been moved back substantially," said the source.

Another commission spokesman Antony Gravili was unable to say whether police had been called in, saying only that an "in-house security team" was probing.

He blamed the "serious attack" on malware rather than any attempt to unearth secret documents relating to summit issues.

"I have no information at all linking the attack to the summit -- we don't only suffer attacks at these times."