Most of the passengers aboard the flight were Chinese, and around a dozen gathered outside the Beijing offices of Malaysia Airlines, with emotions running high.
"I don't believe this latest information about the plane, they have been lying to us from the beginning," said Zhang Yongli, whose daughter was on board.
"I know my daughter is out there, but they won't tell us the truth," he added.
Bao Lanfang, whose grandson was also on the plane, told reporters, "Everyone has been lying to us", before collapsing on the floor and crying.
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The flight, with 239 people on board, vanished en route from Kuala Lumpur to Beijing in March last year.
No evidence had been found until the debris, part of a wing known as a flaperon, washed up on the French territory of Reunion, and Malaysian Prime Minister Najib Razak said early today it was from the jet.
French officials used more cautious language, saying only that there was a "very high probability" the wreckage came from MH370.
Several gathering today held signs with a picture of an aeroplane, reading: "It will surely return safely".
On a social media group other relatives expressed similar sentiments, saying: "Don't believe them! They must have switched the debris! We do believe all our relatives will come back safe and sound!"
Elsewhere Chinese people took to social media sites to express scepticism about Malaysia, whose reputation has taken a knock in China for its handling of the incident.