UKIP's Paul Nuttall has described the decision to launch his party's manifesto as a message to terrorists "that they will not win".
Mr Nuttall said there was no magic formula to "turning back the tide of radical Islam" - but he said it was a "cancer that needs to be cut out".
He said other parties had been too cowardly to address the problem.
Deputy chairwoman Suzanne Evans said Theresa May had "some responsibility" to take for the Manchester bombing.
Mr Nuttall said UKIP would beef up security by increasing numbers of police officers, troops and border guards.