The grenade was tossed off a bridge by assailants on a motorbike at the Barbir intersection and fell near a checkpoint for security forces in the Sunni Muslim neighborhood of the Lebanese capital.
There was no immediate word on casualties, but at least one vehicle was damaged, according to police, speaking on customary condition of anonymity.
Beirut area has seen three explosions in the last week, killing a woman and injuring nearly 30 people.
The violence came as Lebanese troops were fighting Islamic militants at the Palestinian refugee camp of Nahr el-Bared in northern Lebanon. Scores of people have been killed in the northern fighting.
The discovery of the submarine has unveiled a few "inconsistencies." For example, how can one explain the fact that the sub was found where it needed to be searched for from the start?