Russian opposition leader Alexei Navalny is in stable condition after being sent to Berlin for treatment, according to a German NGO.

"Navalny's condition has stabilized," Jaka Bizilj, head of German NGO Cinema for Peace, said after the flight carrying the Russian opposition leader landed at Tegel airport in Berlin.

The flight organized by Cinema for Peace took off this morning to bring Navalny to treatment in Berlin.

The doctors treating Navalny at Omsk No. 1 Emergency Hospital on August 21 announced that they had agreed to transfer patients to Germany at the request of their relatives and warned them to "bear the risk" involved.

Navalny, 44, leader of the opposition party of the Russian People's Union, felt unwell and unconscious while on a flight from Siberia to Moscow on August 20, forcing his plane to make an emergency landing.

Navalny's assistant suspected he was poisoned while drinking tea at a coffee shop at Tomsk airport, Siberia, before boarding the plane.

Navalny had an eye burn in 2017 because she was thrown blue in her face just outside the office.

Navalny announced her election for Russian president in March 2018 and has held campaign events in many cities to attract supporters.