The grenade was thrown by an unknown assailant after a parking dispute between three people in the Kola district, late on Wednesday night, police said.
The sound of the explosion caused panic in the area.
One hour later, 200g of TNT in a plastic bag exploded in an empty field in the Mamura neighbourhood, a mainly Shia area on the outskirts of Beirut's southern suburbs, police sources said.
There were no casualties, police said.
Shortly after the explosion, the Lebanese army set up checkpoints at the southern entrances of Beirut and intensified their patrols across the capital.
The security situation in Lebanon has been volatile since the 14 February assassination of former prime minister Rafiq al-Hariri.
Amid mass protests against the killing, a series of explosions targeted shopping areas in the Christian areas of east Beirut.
Three people were killed and about 20 others were wounded in the series of blasts which started on 19 March.