A policeman was killed and another seriously wounded in the attack in Soweto, near Johannesburg.

The prisoners were being transported in police vehicles from a police station to a nearby magistrate's court when the convoy ran into an ambush at a set of traffic lights.

Of the 23 prisoners travelling to court, all but two escaped.

The incident follows the reported escape of 11 prisoners from a Johannesburg jail on Christmas Day. All were serving long sentences. One, who has been convicted of several murders, had escaped five times before.

Embarrassment

BBC Africa Correspondent Jane Standley says the escapes are an embarrassment to a government which is promising to cut down South Africa's high levels of crime.

However, the South African prison authorities say they have radically reduced the number of escapes in the last two years.

According to official figures, only 480 prisoners escaped last year, compared to 1,069 in 1997.

Police robbed at gunpoint

In an unrelated incident on Sunday morning, five armed men robbed a police station in Claremont, in the suburbs of Cape Town.

The robbers held three policemen at gunpoint and demanded the keys to a safe, from which they stole firearms and cash.