The attacks came after the Anonymous group of hackers pledged to pursue firms that have withdrawn services from Wikileaks.
Mastercard payments were disrupted but the firm said there was “no impact” on people’s ability to use their cards.
Visa’s website now appears to be experiencing problems. The attacks came after both companies stopped processing payments to the whistle-blowing site.
Entries on the Twitter page of Operation Payback, the Anonymous campaign, said the Visa site had been taken down.
“Hackers Take Down Visa.com in the Name of Wikileaks. Wow. This is getting crazy,” read one message on the page.
Paul Mutton at the security firm Netcraft, who is monitoring the attacks, said Visa is considered a more difficult target and the attack on it required a much larger number of activists, 2,000 compared with 400 for Mastercard.
But it appeared to have succeeded, he said, with Visa.com remaining inaccessible to many users.
Earlier the BBC was contacted by a payment firm linked to Mastercard that said its customers had “a complete loss of service”.
In particular, it said that an authentication service for online payments known as Mastercard’s SecureCode, had been disrupted.
/* -1 && userAgent.search(“version/3.”) > -1) &&
!(userAgent.search(“windows”) > -1 && userAgent.search(“safari”) > -1 && userAgent.search(“version/4.”) > -1) &&
userAgent.search(“android”) == -1)
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// PayPal’s Osama Bedier: “Our policy group had to make the decision of suspending the account”
Other readers have also said that they have had problems with online payments. The scale of the problems is still unclear.
Mastercard said in a statement that it was making “significant progress” in restoring full service to its website.
“Our core processing capabilities have not been compromised and cardholder account data has not been placed at risk,” it said.
“While we have seen limited interruption in some web-based services, cardholders can continue to use their cards for secure transactions globally.”
Anonymous, which claimed to have carried out the attack, is a loose-knit group of hacktivists, with links to the notorious message board 4chan.
It said that it has hit several targets, including the website of the prosecutors who are acting in a legal case against Wikileaks founder Julian Assange.
Websites that are bowing down to government pressure have become targets”
End Quote Coldblood Anonymous
An Anonymous member told AFP news agency the group would extend their campaign to anyone with “an anti-Wikileaks agenda”.
PayPal, which has stopped processing donations to Wikileaks, has also been targeted.
The firm said Wikileaks’ account had violated its terms of services.
“On 27 November the State Department, the US government, basically wrote a letter [to Wikileaks] saying that [its] activities were deemed illegal in the United States,” PayPal’s Osama Bedier told the Le Web conference in France.
“And as a result our policy group had to make the decision of suspending their account.
“It’s honestly, just pretty straightforward from our perspective and there’s not much more to it than that,” he said.
Other firms that have distanced themselves from the site have also been hit in the recent spate of attacks including the Swiss bank, PostFinance, which closed the account of Wikileaks founder Julian Assange.
The bank said Mr Assange had provided false information when opening his account.