Passengers on the first Eurostar trains since the accident described the lingering smell of smoke in the tunnel and said the journey took longer than usual.
Freight service resumed overnight, and Eurostar began "limited service" Saturday morning on its passenger trains but warned of longer travel times. Trains are only running in one of two tunnels that normally carry traffic, the one untouched by Thursday's fire. The other tunnel remains closed.
Eurostar says 12 trains are expected to run Saturday in each direction between Paris and London and up to six trains in each direction between Brussels and London.
The blaze broke out Thursday afternoon and was not extinguished until midday Friday. Firefighters worked through the night, enduring extreme temperatures and cramped quarters as they put out the blaze 40 meters under the English Channel.
While investigators were trying to determine the cause of the blaze, the state prosecutor of France's coastal Boulogne-sur-Mer region, Gerald Lesigne, said he believed it was of "accidental origin."
"There is no element in this case that indicates that this fire could have originated from an act of malice," he told The Associated Press.
The fire left the British Isles cut off for more than a day from continental Europe other than by sea or air - the only routes that existed before the tunnel revolutionized travel between France and England when it opened in 1994.
Before Thursday's accident, some 26,000 people traveled through the tunnel on average each day.
Janet Larkin and her family, returning to Britain from a holiday in Spain, had been scheduled to leave Paris on Friday night but had to wait until traffic resumed Saturday.
"When we got in the tunnel it really smelled of smoke, it was horrible," she said. She the train was about 45 minutes late but passengers were given free breakfast.
The tunnel has had several fires in the past, including one in 1996 that shut freight traffic for months.