Hackers identifying themselves with Antisec have attacked the website of Panda Labs' anti-malware products.
It follows yesterday's arrest of an alleged member of the movement alongside others linked to the Lulzsec hacking collective.
The site's front page was replaced with a message saying "Love to Lulzsec/Antisec fallen friends".
It accused the firm of having helped police arrest other hackers last month. The company denies the claim.
The group Lulzsec and hackers involved with "Operation Antisec" all identify themselves under the wider Anonymous heading.
Panda Lab's Spanish parent company, Panda Security, said it was investigating the intrusion but said it could assure its customers that none of their information had been compromised.
It added that an external server hosting some of Panda Labs' content had been targeted. Visitors to the site no longer see the hackers' posting.
The hackers' message had claimed that: "Pandasecurity.com... has earned money working with law enforcement to lurk and snitch on Anonymous activists. They helped to jail 25 Anonymous in different countries and they were actively participating in our IRC channels trying to dox [obtain personal information about] many others."
The attackers went on to claim that the firm's services had been used to target activists campaigning against "injustices" who had not been involved in hacking.
However, Panda Security's technical director, Luis Corrons, told the BBC that the firm had not been involved in February's arrests in Spain, Argentina, Chile and Colombia.
"If I could have had the opportunity I would love to have collaborated with the law enforcement officers to arrest those people - but we didn't in this case," he said.
"We do not have a problem with activism. I find that it is not a bad thing - it may be good and healthy for a society. My main problem with Anonymous people is the way they act.
"If you are doing illegal things like stealing information that's something that is a crime and we are always willing to help law enforcement stop cybercrime."
Mr Corrons said he believed his firm had been attacked because of a blog he wrote yesterday in which he described the latest arrests as "good news".
His comments were quoted in the message posted to his firm's site which added: "Lol he asked for the Lulz!!!"
Officials have said that Tuesday's action marked the first time core members of Anonymous had been identified and charged in the US.
The FBI said a total of six men had been charged - including two in the UK and two in the Republic of Ireland.
British police also charged one of the men and a further unidentified 17-year-old from South London with hacking-related crimes - but said that their decision to launch court proceedings was "completely unrelated" to the actions taken in the US.
'Sad' about Sabu
The arrests and accusations followed the suspected leader of Lulzsec's decision to co-operate with the authorities last year.
Court papers unsealed on Tuesday revealed that Hector Xavier Monsegur - known as Sabu - had pleaded guilty to 12 criminal charges in August. The FBI said hefaced a maximum sentenceof more than 124 years in prison.
The papers said he had been involved in attacks against Visa, Paypal and government computers in Tunisia, Algeria and Yemen among others. He is currently free after being released on a $50,000 (£31,760) bond.
The Panda Labs' attackers commented on Mr Monsegur's action saying: "It's sad and we can't imagine how it feels to look at the mirror each morning and see the guy who shopped their friends to police."
The court papers also revealed how the hackers are suspected of being able to access and disclose a private conversation between the FBI and Scotland Yard recorded in January.
One of the accused, Donncha O'Cearrbhail, is said to have found out how to access the call by breaking into the personal email account of an officer with Ireland's national police force.