Major-General Sarkis Tadros quoted an explosives expert as saying the bomb, which was placed between a car and a building containing a wood factory, weighed about 25kg.
A Lebanese woman and two Indian workers were wounded, he said.
The blast was the third to strike Christian neighbourhoods in Lebanon since last Saturday and comes at a time of political turmoil after the 14 February assassination of former prime minister Rafiq al-Hariri, and the subsequent withdrawal of Syrian troops to eastern Lebanon and Syria.
A bomb on Wednesday killed three people in Kaslik near the port city of Jounieh in the Christian heartland, five days after a car bomb tore through the front of an apartment building in Beirut, wounding nine and raising fears that agitators were trying to show a need for Syria's military presence in Lebanon.
"They must love us - we got it twice in a week"
mayor of Bouchrieh
Antoine Gebara, mayor of the northeastern Beirut area of Bouchrieh, where the explosion took place, said police had been tipped off about a car bomb earlier in the evening and were looking for a vehicle.
However, he said the explosion was the result of a bomb placed at the site.
"It appears it is an explosive charge that was placed there," Gebara told the leading Lebanese TV station, LBC.
"They must love us - we got it twice in a week," he said, referring to an explosion in the nearby predominantly Christian neighbourhood of Jdeideh last Saturday that injured nine people.
Witnesses said the blast, coming on the eve of the Easter holiday, occurred in the Bouchrieh industrial zone, one of Beirut's largest, and happened three hours before Catholics were to head to a midnight mass to celebrate Easter.
An AP photographer at the scene said at least six industrial buildings were ablaze.
Firefighters battled to contain flames leaping from blackened factory windows into the night-time sky. The police chief said highly flammable material in the factories helped spread the fire from one building to another.
Civil Defence officers and the Red Cross urged people to stay away from the area, fearing the spread of fire and more explosions caused by flammable materials and fuels in the factories.
More violence predicted
The motive behind the attacks were not immediately clear, but Lebanese opposition leaders have blamed Syrian security agents and pro-Damascus Lebanese authorities of trying to sow fear in the community.
Lebanese and Syrian authorities have denied such claims.
Druze opposition leader Walid Jumblatt held pro-Syrian Lebanese security agents responsible for Saturday's blast, saying they were trying to intimidate people.
Jumblatt said he expected more car bombs in the coming days and in the run-up to parliamentary elections scheduled to be held by May.