Dutch authorities said they arrested a 16-year-old boy suspected of being involved in attacks on the websites of MasterCard and Visa that were allegedly mounted by sympathizers of document-leaking website WikiLeaks.

Police arrested the suspect at his home in The Hague late Wednesday night and said he had acknowledged being involved in the attacks on the websites of the two credit-card companies, according to Wim de Bruin, a spokesman for the Dutch prosecution office.

Police...