The Islamic State propaganda agency Amaq claimed one of its "soldiers" carried out a van rampage on a crowded street in Barcelona, though the suspect was still at large. Thirteen people were killed.
A second similar attack took place early Friday in the Spanish seaside resort of Cambrils. One of six civilians injured later died. Police killed the five attackers in a gun battle.
"Radical Islamic Terrorism must be stopped by whatever means necessary! The courts must give us back our protective rights. Have to be tough!" Trump said in one of his signature early morning tweet storms.
The Republican billionaire leader blamed "Obstructionist Democrats" for complicating security in the United States.
"They use the courts and associated delay at all times. Must stop!" he said, in a possible reference to the stalling in federal courts of the full implementation of his controversial travel ban on nationals from six mainly
"Homeland Security and law enforcement are on alert & closely watching for any sign of trouble. Our borders are far tougher than ever before!"
On Thursday, Trump courted controversy in the wake of the Barcelona van attack when he appeared to endorse the idea of mass executions for Islamist extremists.
As justification, he alluded to a widely debunked account of summary punishment by a US general in the Philippines in the early 1900s.