The main Paris ring road was slowed to a crawl on Monday morning as hundreds of ambulances, sirens blaring, protested at changes to conditions.
Some 500 ambulances had already gathered in Paris before rush hour on Monday as part of a day of action.
A new regulation means that hospitals and clinics - rather than individual patients - will now choose which ambulance service to use.
Many small operators say that means they will no longer be able to compete.
French media report that up to 2,000 ambulances are expected in total throughout the day.
News outlet France Bleu reported that there were some 400km (250 miles) of traffic jams in the Ile-de-France region by 09:00 local time (08:00 GMT). The A4 motorway was also affected.
Video of the protests posted on social media showed a small army of vehicles at a near standstill on the usually busy main road - each with their lights flashing and sirens on.
Many bore messages against the controversial reforms on their sides and windows, and some paramedics left their vehicles to walk down the road bearing placards.
Police also arrived at the scene in an attempt to clear the blockages.
The action is scheduled to continue until about 16:00, when the paramedics are due to meet in front of the ministry of health.
The reform of ambulance and other health-related transport came into effect on 1 October, which changed the way paramedics were paid for their work.
Instead of patients choosing their provider, hospitals and other medical facilities will arrange transport for patients and pay for it.
Many small independent firms say they will not be able to compete in a public tender process against much larger operations, which may offer bulk discounts.
One paramedic, speaking to BFMTV, said that his firm was already receiving far fewer calls than it used to.