SÃO PAULO—Hackers on Thursday kept up their campaign to cripple Brazilian banking websites and, in a new twist, their efforts appeared to affect both local and global websites of U.K.'s HSBC Holdings PLC.

This is the fourth attack in as many days by the Anonymous Brasil group, which says the effort is part of a campaign aimed at social activism in Brazil, and not theft. Earlier this week, the attacks hampered operations on the websites of Banco do Brasil SA, Itaú Unibanco Holding SA and Banco Bradesco SA.

"Bonus:...