Protesters in Paris held placards reading: "Sarkozy, Don't Touch our Pensions!" while others held a cardboard coffin marked: "Here lies Roger. He's 60, and he died before getting his retirement".
The day of action is a key test of strength between the powerful unions and Nicolas Sarkozy, the French president, who needs to cut a ballooning budget deficit and public debt to help the country maintain its AAA sovereign debt rating.
Estelle Youssouffa, Al Jazeera's correspondent in Paris, said: "The turnout has been massive today, nationwide, with 200 demonstrations taking place across various cities.
"Hundreds of thousands have been taking to the streets here in the French capital.
"Protesters are very angry at the French government's decision to push the retirement legal age to 62.
"The French feel that this reform is unfair, that it is targeting pensioners who are already suffering, and it is one major social benefit that the French don't want to lose."
Six out of France's seven unions joined forces for Thursday's rallies, and said the demonstrations were a foretaste of the protests they plan for September, when the national assembly will vote on changes to the retirement age.
The government has proposed raising the retirement age from 60 to 62, which although lower than in many European countries, breaks a significant taboo in a country where many saw retirement at 60, introduced by a socialist government in 1983, as a human right.
The government unveiled its planned overhaul of the pay-as-you-go pensions regime last week, saying that without major changes the system would run up annual deficits of $134.2bn by 2050.
Transport authorities said about one in two mainline trains ran in and out of Paris on Thursday morning, with three in four Paris metro trains operating.
The DGAC airport authority said 15 per cent of flights were cancelled between 0500 and 1200 GMT, while one in two teachers was expected to go on strike, the teachers' union said.
Radio stations, such as all-news France Info, played music to fill the gaps in their programming left by striking staff.