Facebook confirmed it had removed the activists' Operation Payback site on Thursday because it was promoting a distributed denial of service attack -- an illegal form of freezing websites. Twitter declined to comment.
The swoop against Operation Payback's self-described campaigners for Internet freedom followed their online attacks on credit card giants like Visa and MasterCard.
The campaign reappeared on Twitter later in the European afternoon using another account. Experts said the outages were unlikely to have much effect on the pro-WikiLeaks cyber campaign as activists were using separate chatrooms to organize.
A representative of one of the groups involved in the online campaign said on Thursday that more cyber attacks in reprisal for attempts to block the WikiLeaks website were likely.
On Thursday, supporters of WikiLeaks were plotting attacks on online payment service PayPal and other perceived enemies of the publisher, which has angered U.S. authorities by starting to release details of 250,000 confidential cables.
Amazon was also cited as a target.
"The campaign is not over from what I've seen, it's still going strong. More people are joining," a spokesman calling himself "Coldblood" told BBC Radio 4. The speaker, who had an English accent, said he was aged 22 and was a software engineer.
NEITHER "VIGILANTES" NOR "TERRORISTS"
"Anonymous has targeted mainly companies which have decided for whatever reason not to deal with WikiLeaks. Some of the main targets involve Amazon, MasterCard, Visa and PayPal."
The websites of credit-card giants MasterCard and Visa have already been brought down through distributed denial-of-service attacks that temporarily disable computer servers by bombarding them with requests.
In a statement on Thursday, MasterCard said although there was a limited interruption of some online services, cardholders could continue using cards for transactions worldwide. Its main processing systems were not compromised, the statement said.
AnonOps also claimed responsibility for bringing down Visa Inc's site, which was temporarily unavailable in the United States, but later restored.
In an online letter, Anonymous said its activists were neither vigilantes nor terrorists. It added: "The goal is simple: Win the right to keep the Internet free of any control from any entity, corporation, or government."
Swedish newspaper Aftonbladet said the Swedish government's website was down for a short time overnight in the latest apparent attack.
Sweden has issued an arrest warrant for WikiLeaks founder Julian Assange over sex crimes and he is in jail in London, awaiting an extradition hearing.
Assange, a 39-year-old Australian, has been hailed as an advocate of free speech by supporters, but now finds himself fighting serious sexual allegations made by two women in Sweden.
Assange will have another court appearance next Tuesday and his supporters assert he is being victimized for his work.
MORE SECRET CABLES RELEASED
In the Internet Relay chat channel where activists coordinated the attacks, conversations were short and to the point. Participants asked what the target should be and reported progress. Some bemoaned the fact that paypal.com remained up despite efforts to bring down its transactions server.
"The only thing most of these CEOs understand is the bottom line. You have to hit them in the bank account, or not at all," said one participant called Cancer.
WikiLeaks is continuing to drip-feed cables into the public domain despite the legal woes of its founder.
Those released on Thursday showed U.S. diplomats reporting that the illicit diamond trade in Zimbabwe had led to the murder of thousands, enriched those close to President Robert Mugabe and been financed in part by the central bank.
Assange's online supporters hit the corporate website of MasterCard on Wednesday in reprisal for its blocking of donations to the WikiLeaks website.
"We are glad to tell you that http://www.mastercard.com/ is down and it's confirmed!" said an entry on the Twitter feed of a group calling itself AnonOps.
"Coldblood" said a battle was under way to protect the Internet. "I see this as becoming a war, but not your conventional war. This is a war of data. We are trying to keep the Internet free and open for everyone, just the way the Internet always has been," "Coldblood" added.
Assange's main London lawyer has denied that the WikiLeaks founder ordered the attacks.
"It's very hard to get hold of anyone from WikiLeaks. The only (person) you could really get hold of was Julian, but unfortunately he's not available at the moment," said "Coldblood."