Video games publisher Ubisoft has apologised after thousands of players were locked out of its systems over the weekend.

PC users started reporting problems accessing some of the French company's most popular games, including best-seller Assassin's Creed 2, on Sunday afternoon. It later emerged that attackers had targeted the company's controversial anti-piracy system, causing it to break down - which in turn left thousands of people unable to play.

The chaos was so widespread because of the way that Ubisoft's copy protection system - which requires players who have bought the game to log in online and verify that they are not playing a pirated version - is designed. By flooding the anti-piracy servers with web traffic, the unknown attackers forced it to collapse and therefore locked out those players who tried to sign in.

This angered many gamers, who felt that they had been punished for buying legal copies of the company's games - which cost as much as £50.

"We've had to agree to their draconian rules in order to play their game, however Ubisoft haven't given a single thought to what happens when their servers screw up," said one disgruntled user on the company's web forums.

"[This] only penalises legitimate customers like myself who want to play your fantastic games but cannot," said another.

The company initially blamed the problems on "exceptional demand" - but in a statement on Monday, Ubisoft admitted that it had been targeted.

"Ubisoft would like to apologise to anyone who could play Assassin's Creed 2 or Silent Hunter 5 yesterday," it said. "Servers were attacked and while the servers did not go down, service was limited from 2.30pm to 9.30pm Paris time."

"Ninety-five percent of players were not affected, but a small group of players attempting to open a game session did receive denial of service errors."

The attack itself brings into question the company's decision to roll out its controversial digital rights management (DRM) to try and stop its games from being illegally copied.

Although the games themselves do not involve online play, the sign-in system is required for anybody playing titles like Assassin's Creed 2, which has sold more than 8m copies worldwide since it was release in November.

The constant tension between publishers, consumers and pirates has caused problems in the past - such as when Electronic Arts released Spore, a highly-anticipated evolutionary simulator that required online validation before it could be played. After complaints from thousands of users, the company eventually relaxed the rules.

But Ubisoft's system has caused extra irritation with players because it means that the titles cannot be used at all without a constant internet connection - a particularly drastic requirement given that the company says there are actually no cracked versions of the games in question in the wild.

The same denial of service technique employed by the Ubisoft attackers is often used by hackers and blackmailers to threaten online businesses, or by those protesting against web sites or companies.

And despite the company's assurances that only a handful of players were affected, however, it appears that the problems may not yet be over.

On Monday some users reported similar difficulties, leading the company to confirm via Twitter that "our servers are under attack again" and that "we're working on it".