The first concrete slabs forming the walls of the new Sta Lucija tunnels were put in place this week, Infrastructure Minister Ian Borg said.
The tunnels, which are being dug rather than bored, will link Vjal Santa Luċija with Triq tal-Barrani.
The €20m project is being carried out in parallel with the Marsa Junction project works. There will be two two-lane tunnels more than 100 metres long, with a new roundabout built atop them and newly-laid water, electricity and telecommunications infrastructure.
The project was approved by the Planning Authority on September 18 but drew criticism because of the destruction of mature trees.
It is scheduled to be completed by the end of this year.
How the tunnels will look on the project's completion.