An 18-year-old man has been arrested on suspicion of terrorism offences and possession of weapons.
Portuguese police say they have foiled a planned attack on students at the University of Lisbon's science department.
An 18-year-old man was arrested on Thursday on suspicion of terrorism offences, judicial police said in a statement.
He is also accused of illegally possessing weapons, police added.
Investigators say they seized several banned weapons and found "vast elements of evidence" including "a written plan with details of the criminal action".
According to national media, Portuguese authorities were alerted by foreign officials who found evidence of the planned attack.
"The investigation was triggered by suspicions of an attack on university students at the University of Lisbon," the police statement read.
"Given the seriousness of the suspicions, the highest priority was given to the investigation, which allowed, in the early hours of today, to interrupt the ongoing criminal activity," it added.
The suspect's motives for an attack are unclear and he held no criminal record, police added. The 18-year-old will appear for questioning on Friday.
"The work carried out by the authorities has always ensured the safety of the scientific community," said a spokesman for the University of Lisbon.
"There is no indication that it is necessary to change the normal functioning of the institution," they added.