The 270km (167-mile) high-speed line is due to link the French city of Lyon with Turin in northern Italy at an estimated cost of €26bn ($30.2bn).
The plan has been beset by disapproval and protests, both regional and national, and in July Deputy Prime Minister Luigi Di Maio, leader of the M5S and industry minister, called for a review, putting his group at odds with coalition partner the League.
Around 30,000 people gathered in one of Turin’s main squares for what was the biggest pro-TAV demonstration so far, signalling the plan’s economic importance in one of Italy’s most industrialised regions.
“It’s yes, to economic development, to jobs, tourism and culture and all that is important to the city,” said Adele Olivero, one of the organisers of the rally, which gathered citizens, labour unions, parties as well as TAV workers.
Regional Governor Sergio Chiamparino called for a quick response from the government.
“Turin and (the region) of Piedmont want the TAV, and they want a link with Europe, without barriers,” he said.
Chiara Appendino, M5S mayor of the city, said she accepted the arguments put forward by the protesters and that she was ready to “open a constructive dialogue”.
Work has started on the project but Italy’s infrastructure minister said he wanted to re-negotiate the terms with France.
If Italy backs out at this point, it could face large penalties from both Paris and Brussels.
The M5S has long supported the cause of residents of the alpine valleys who have protested against TAV.
Meanwhile, the League, whose support is mainly rooted in the regions of the north, has defended the project, saying it will help develop the Italian economy.
M5S, afraid to lose a large slice of its electoral support, is keen to stop works on TAV, one of its biggest election promises, particularly after having to grant approval last month to the Trans Adriatic Pipeline (TAP) project, which it had highly contested and had pledged to stop.
