French authorities on Monday asked unions to scrap plans for a protest march in Paris and limit their demonstrations to a single, static rally in hope that the violence which marred previous days of action can be avoided.
The government has the right to ban the protest rally if unions do not comply with its demands.
Tension between the government and unions that oppose the reform, including the hardline CGT, rose sharply after an escalation in violence during a street march in Paris last week.
Hundreds of rioters ransacked shop fronts, clashed with police, tore up street paving and smashed the windows of a children's hospital during running battles on Tuesday. The police responded with teargas and water cannon and dozens were hurt on both sides.
"In this context of tension and recent clashes ... having another march like that on June 23 is not conceivable," Interior Minister Bernard Cazenove said in a letter to CDT chief Philippe Martinez.
A Paris police chief had made a similar comment earlier on Monday, but two top union officials told Reuters they would stick to their plans.
Unions say the violent acts are committed by people not connected with their movement.
"I don't see what could push us to give up our constitutional right to protest," Pascal Joly, the head of the CGT in the Paris region, told Reuters.
Rallying in one square instead of doing a protest march "is not acceptable," said Gabriel Gaudy, a top official for smaller union FO.
French police are already stretched as France hosts the Euro 2016 soccer tournament under a state of emergency in place since Islamist attackers killed 130 people last November.