Most of the migrants were Somalis and Ethiopians and included "pregnant women", said Mohammed Abdelsalam al-Kuwiri, spokesman for a unit in the Tripoli-based government that combats illegal migration.
He said they were arrested as they were getting ready to board boats in Tajura, east of the capital.
An AFP photographer said dozens of migrants were taken by car to a detention centre in Tripoli.
A migration squad official said the arrests coincided with the launch of an operation targeting people smugglers.
The operation, run by the Tripoli-based government which is not recognised by the international community, calls for "coordination between various security services", the official said.
The government in Tripoli was created by the powerful Fajr Libya militia alliance last August during a power struggle with the internationally recognised administration now based in the east.
Fajr Libya is one of several heavily armed militias that have been vying for power in Libya since the end of the 2011 uprising that toppled veteran dictator Moamer Kadhafi.
Jihadists from the Islamic State group have also gained a foothold in the oil-rich North African country, feeding on the political breakdown and lawlessness.
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People smugglers have operated in Libya for years, but have exploited the chaos since the end of the revolt to step up their lucrative trade.
With a coastline of 1,770 kilometres (more than 1,000 miles), Libya has always been a stepping stone for Africans seeking a better life in Europe.
Most head for the Italian island of Lampedusa 300 kilometres (185 miles) from Libya's shores.
One man who had hoped to reach Italy before his arrest Sunday was Adam Ibrahim Abdullah from Somalia.
"I paid $1,400 to come to Tripoli. I stayed here in a compound for two months, and then I paid $1,400 again to go to Italy," he told AFP.
He said he wanted to start a new life in Italy "because my country is in conflict and there is no government".
Maneh from Niger and Anabelle from Nigeria said they hoped to join relatives who have already left for Europe, pleading not to be deported back home.
"I don't want to go back to my country. There is nobody to help me there. I beg you, keep me in Libya," Maneh said, tears running down her cheeks.
Illegal migrants arrested in Libya are usually held in detention centres before being deported.