"This attack was particularly heinous as perpetrators have themselves claimed that they targeted the Christian community who had gone to the park on Easter Sunday.
"The religious extremists who are resorting to such terrorist violence have to be put down firmly by the Pakistani authorities," the party said in a statement.
At least 72 people, mostly women and children, were killed, while 300 others were injured when a suicide bomber blew himself up in the capital of Pakistan's Punjab province.
