An alleged people smuggler was also detained.
An early morning raid by forces battling clandestine migration targeted a house in eastern Tripoli's Al-Hashan district and netted several dozen people, an AFP journalist said.
Armed and masked members of the authorities in bullet-proof vests rounded up the migrants and put them on vehicles to be driven to a detention centre.
Dozens more were detained in another raid on a house in the same neighbourhood.
No force was used in either operation and there was no resistance.
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"We arrested 203 illegal migrants from different nationalities. They were with there with their smuggler," he said.
Libya has long been a stepping stone for migrants seeking a better life in Europe, with Italy some 300 kilometres away across the Mediterranean.
Smugglers have stepped up their lucrative business in the chaos that has followed the 2011 ouster of longtime dictator Moamer Kadhafi.
The interior ministry in Rome put migrant arrivals in Italy at 23,739 since the start of the year as of Thursday morning, compared with 19,589 by April 14 last year.