It is not clear when lorries will be allowed through again
A white convertible car with British licence plates was at the head of about 50 vehicles waiting for the green light to show after a religious benediction for the fully rebuilt tunnel under the Alps.
Just hours earlier, a device - believed to be a grenade used to start avalanches - exploded on an approach road on the French side of the tunnel, which goes to Italy under western Europe's highest mountain.
A maintenance truck was damaged but no one was hurt and the tunnel was reopened at around 1150 GMT, nearly an hour behind schedule.
The opening ceremony was sombre and dedicated to the victims of the March 24, 1999 fire.
After the blessing, Gilbert Santel, president of Mont Blanc Highway and Tunnel which runs the French side, hailed the reopening as a "symbol of free circulation in Europe."
Nearly 200 police reinforcements were brought in as 1,500 activists opposed to goods traffic gathered for a protest.
Pollution
The demonstrators - ringing cow bells and carrying posters - marched from the nearby town of Chamonix to the tunnel's French entrance.
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Environmentalists are concerned about the return of heavy goods traffic and the consequent pollution.
Initially only cars will be allowed through the tunnel, according to a French-Italian government committee which announced the reopening following final security checks this week.
Lorries are expected to be allowed through the tunnel after mid-March.
Heavy goods traffic will be staggered - with vehicles only allowed through the tunnel in one direction at a time - and lorries carrying dangerous substances will not be allowed into the tunnel.
Hauliers say the plans will lead to congestion either side of the tunnel, and accuse the French Government of pandering to public fears resulting from the fire.
The 1999 fire burned for two days and devastated the 12-kilometre tunnel.
It started in a lorry then spread to nearby cars, producing choking smoke and extremely high temperatures.
The inquiry into the fire is continuing.
Alpine disruption
The tunnel is one of the major trans-Alpine routes and its closure has caused congestion on other tunnels and mountain passes.
It has particularly affected the Italians, who rely on the tunnel for transporting goods to northern Europe.
An estimated one-third of Italy's freight used to pass through the tunnel.
The closure of the Gotthard Tunnel in Switzerland after a fire there last autumn exacerbated the problems.
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