A multinational search effort has focused on seas some 2,500km (1,500 miles) to the south-west of the Australian city of Perth. Flight MH370 disappeared on 8 March as it flew from Kuala Lumpur to Beijing. It was carrying a total of 239 people, including 153 Chinese nationals.
In Beijing, relatives of passengers on board the plane released a statement accusing the Malaysian government of trying to "delay, distort and hide the truth".
Dozens of them then left their Beijing hotel on a protest bound for the Malaysian embassy, carrying banners asking Kuala Lumpur to be truthful with the relatives.
Police blocked their buses from leaving, so they left the buses and walked there themselves, with scuffles then erupting outside the diplomatic mission.
The protesters threw water bottles at the embassy and tried to storm the building, demanding to meet the ambassador.
There was a heavy police presence at the embassy and there was a brief scuffle between police and a group of relatives who tried to approach journalists, according to AP news agency.
The clashes in Beijing came a day after the Malaysian leader said it had to be concluded "with deep sadness and regret" that according to new data "flight MH370 ended in the southern Indian Ocean".
Mr Najib said the conclusion the plane was lost was based on new satellite analysis by British firm Inmarsat and information from the UK's Air Accidents Investigation Branch (AAIB).
Chinese Deputy Foreign Minister Xie Hangsheng issued a statement saying "We demand the Malaysian side state the detailed evidence that leads them to this judgement as well as supply all the relevant information and evidence about the satellite data analysis. BBC News.