Michel Aoun spoke on Friday after the number of victims in the explosion rose to 154, with another 5,000 people injured.
"There is a possibility of external interference through a rocket or bomb or other act."
International rescue teams are still searching the rubble of Beirut's port nearly three days after the explosion hit the city.
Lebanon's health minister later said at least 154 people had died in the blast on Tuesday.
French and Russian rescue teams with dogs were seen searching the port area on Friday.
Among those located in the rubble was Joe Akiki, a 23-year-old port worker who had been missing since the explosion.
Dozens of people are still missing.
Some 300,000 people, more than 12% of Beirut's population, are unable to return to their homes because of the explosion, which blew out doors and windows across the city and left many buildings uninhabitable.
Officials have estimated losses at $10bn to $15bn (£7.6bn to £11bn).
https://news.sky.com/story/beirut-explosion-lebanons-president-says-blast-may-have-been-caused-by-a-rocket-or-a-bomb-12044405