The bomb went off just after 8pm, close to a cinema and a crowded cafe, as residents were breaking their daily fast for the holy month of Ramadan. The explosion, in the busy Belcourt district of the Algerian capital, could be heard right across the city.
The government said 10 people were killed, but hospitals receiving the victims gave a casualty toll of at least 20 dead and 60 or more wounded, many seriously. "I counted at least 10 bodies of dead people and I saw more than 50 wounded," one witness said.
Ambulances and police cars converged on the area, and the authorities sealed off the area close to the popular Cinema Musset.
Another explosion was reported last night near Reghaia, about 18 miles east of Algiers. There were reports of injuries and damage.
Earlier the security forces said in a statement carried by the official Algerian news agency APS that 36 people were killed in the village of Sidi Abdelaziz in Medea province, a Muslim guerrilla stronghold, 40 miles south of Algiers.
The killing was the worst in a series of massacres, mostly in remote villages, and bombings in Algiers and surrounding towns in which more than 300 people have died in the past two-and-a-half months.
No one immediately claimed responsibility for either attack, but suspicion fell on Islamic radicals fighting to overthrow the government and install strict Koranic rule.
The security forces blamed the massacre on a "group of terrorists", the term the authorities use to describe Muslim guerrillas in Algeria.
It was the latest example of bloodshed in the North African country which has been wracked by a five-year Islamic insurgency that has claimed at least 60,000 lives. The conflict began in Algeria in January 1992, when the army-backed government cancelled legislative elections that candidates of the Islamic Salvation Front were expected to win.