The European parliament website came under cyber attack Thursday but hackers failed to penetrate the assembly's internal network, a spokesman said.
"This morning the parliament's site came under attack with massive consultation bids," spokesman Jaume Duch told AFP.
"But the hackers were unable to penetrate the parliament's inner network which continued to operate normally internally," he said, adding that an inquiry was under way.
The attack came the day of the signing in Tokyo of the Anti-Counterfeiting Trade Agreement (ACTA), which aims to create international standards for intellectual property protection, but some groups oppose it as limiting Internet freedoms.
The pact has has sent thousands of protesters into the street this week in Poland, where dozens of websites shrouded their pages in black in protest.
However, there was no mention of an attack on the European parliament by the hacking group Anonymous.
Its Facebook page said: "Mad about ACTA? Focus actions on the EU Commission (DG Trade) & Council of EU, NOT the European Parliament -- they can still say no to ACTA."
Members of the parliament have not yet voted on the agreement and might not tackle the issue until their June session.
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