Mr. Ko Ni, 65, a Muslim and a member of the ruling National League for Democracy, was returning from Indonesia with about 20 other government officials and civic leaders, who had traveled there as part of a government-organized trip to discuss democracy and conflict resolution.
He was shot in the head at close range as he was about to leave the airport in his family car, according to witnesses.
"During the shooting, he was holding his grandchild," said U Aung Myint Oo, an airport security guard. "He fell down bleeding on the ground and died on the spot."
According to taxi drivers who witnessed the attack, the gunman shouted, "You can't act like that," before opening fire.
Police were seen searching the house of Mr. Kyi Lyn in a neighborhood of Mandalay.
No motive for the murder has been given. Mr. Ko Ni was one of the best-known Muslims in Buddhist-majority Myanmar, serving as a legal adviser to the N.L.D. He was the author of six books on human rights issues and democratic elections, and was actively involved in the interfaith peace movement.
"It seems the gunman knew the exact time of his arrival and was waiting to shoot him," said a member of the team who traveled with Mr. Ko Ni to Indonesia, and who spoke on the condition of anonymity over concern for his safety. "I was shocked and scared. It is unsafe here."
A spokesman for the N.L.D., U Win Htein, said during a telephone interview from Naypyidaw, Myanmar's capital, that Mr. Ko Ni was a key adviser in recent years to Ms. Suu Kyi, the former opposition leader turned leader of Myanmar, on constitutional amendments.
"His assassination was a big blow to the National League for Democracy, and it would be very difficult for us to replace him," Mr. Win Htein said. "We lost a hero. It is a bad situation here."
Amnesty International, which worked with Mr. Ko Ni on human rights issues in Myanmar, called for an independent investigation into his death.
"The killing of prominent lawyer U Ko Ni in Yangon today is an appalling act that has all the hallmarks of an assassination," Josef Benedict, the organization's deputy campaigns director for Southeast Asia and the Pacific, said in a statement.
"It demands that the authorities immediately launch a thorough, independent and impartial investigation," Mr. Benedict added. "The authorities must send a clear message that such violence will not be tolerated and will not go unpunished."