Ko Ni was the Supreme Court advocate for the NLD and a long-standing adviser to the country's leader Aung San Suu Kyi.

The unidentified gunman made a close shot to the head as Ko Ni walked out of the arrival gate at Rangoon's airport, the Ministry of Information said in a video posted on state-run MRTV.

A friend of Ko Ni who witnessed the shooting said the suspected assassin also shot a taxi driver who tried to stop him from fleeing.

"As I stopped my car by the airport, that's where I saw Ko Ni's body lying on the walkway outside of the airport and I couldn't believe that just happened," said Thet Paing Soe, a friend and an NLD supporter.

"Then, as the shooter tried to run away, the police arrested him."

Thet Paing Soe said a taxi driver known as Nay Win stopped the fleeing assassin but was shot. The taxi driver's condition was not clear.

The Ministry of Information identified the suspect as Kyi Linn from Mandalay. The motive was not known.

Kyee Myint, a former chairman of the Myanmar Lawyer Network who has a close relationship with Ko Ni, also confirmed his death.

"It is a big loss for us that Ko Ni, our beloved friend, has been killed. He is the face of the democracy in our country and this is a big loss for us," Kyee Myint added.

Ko Ni was Burmese and one of the NLD's most prominent Muslim members.

He criticised the party in 2015 for not putting up Muslim candidates in the general election.

Burma is a mainly Buddhist country and anti-Muslim sentiments have increased in recent years following deadly violence in the western state of Rakhine home to many Rohingya Muslims.

As a practicing lawyer, Ko Ni had handled more than 900 criminal cases and more than 1,400 civil cases. He established the Laurel Law Firm with two other advocates in 1995.