vessels continued their hunt in darkness with the aid of searchlights.
The identity of the victims, missing and rescued could not immediately be established, apart from one Moroccan pulled alive from the sea.
Tunisian authorities said disaster struck overnight Thursday after some 250 would-be immigrants to Europe from various parts of Africa had embarked on a beach apparently on the coast of neighbouring Libya.
The causes of the sinking were not immediately clear, but local inhabitants were quoted as saying the boat was overloaded and capsized in a high wind southeast of the Tunisian industrial town of Sfax between the islands of Kerkennah and Djerba.
The alarm was first raised when small fishing port of Mahdia near Sfax received a radio distress signal at dawn Friday from the skipper of a fishing vessel from Sfax.
Three police and naval patrol boats were leading the search out at sea on Friday night.
Four trawlers and tugs from nearby oil terminals also took part in rescue operations.
The immigrants and were headed for Italy when the boat sank in international waters, some 60 nautical miles southeast off Sfax on Tunisia's eastern coast, the official TAP news agency said.
Thanks to its proximity to the Italian coast, Tunisia has become, along with Morocco and its proximity to Spain, a favourite crossing point for clandestine immigrants from Sub-Saharan and North Africa headed for Europe.
In a recent upsurge in clandestine migration in the region, another boat carrying 28 people bound for Italy was intercepted Friday off Zarzis, further south along Tunisia's east coast. Late Thursday a makeshift boat carrying 24 Africans from a "neighboring country" -- again likely Libya -- was stopped off Tunisia's Djerba island.
The Italian authorities on Wednesday accused Libya of the point of departure for boats ferrying immigrants across the Mediterranean to Italy's shores. A week ago a boat that had sailed from Libya with 70 people from north and sub-Saharan Africa sank near the Italian island of Lampedusa, claiming at least seven lives.
The Tunisian authorities have clamped down on a recent increase in attempts to reach European shores.
Tunisian and Italian authorities cooperate closely in the fight against people trafficking, and have signed agreements allowing quotas of migrant workers to enter Italy.
The Tunisian Human Rights League has stressed the need to identify the economic and social reasons that push thousands of young people to put their lives in such danger.
The League criticised what it calls the selective policies of European nations, which it accuses of closing their borders to people from the southern Mediterranean while encouraging those same countries to open up to European goods and capital.