Westbound traffic has been diverted via the A48 Southern Distributor Road between M4 junctions 24 and 28.
The westbound carriageway, between Junction 25a and Junction 26 of the M4, will reopen at 06:00 BST on Monday.
Motorists have been warned there is "stop-start traffic" on M4 Westbound between the Coldra (junction 24) and Caerleon (junction 25).
Inrix traffic sensors are also showing slow traffic on the A48 Southern Distributor Road to the south of Newport.
Both tunnels will be closed during the night occasionally as engineers upgrade the mechanical and electrical systems.
Scheduled weekend closures for the Brynglas Tunnels on the M4:
Westbound:
Eastbound:
The westbound side of the M4's Brynglas Tunnels at Newport is closed on Sunday as its 18-month upgrade continues.