This is reported by BFMTV with reference to the police.

EN DIRECT - Gilets jaunes: 20 interpellations à Paris pour port d'arme prohibée https://t.co/g6GaEnJUrFpic.twitter.com/AkDA0DkgYT

- BFMTV (@BFMTV) January 19, 2019

According to the TV channel, they were detained for carrying prohibited weapons.

Earlier it was reported that in connection with the regular protest of "yellow vests" in the country mobilized about 80 thousand law enforcement officers.