A portrait of Boris Nemtsov and flowers are pictured at the site where he was killed on February 27, with St. Basil's Cathedral seen in the background, at the Great Moskvoretsky Bridge in central Moscow March 6, 2015. [Photo/Agencies]

Past high-profile killings in Russia have led to people being jailed for pulling the trigger -- often hired hitmen from the Caucasus -- while investigators have failed to track down those who ordered the assassinations.

"I want to believe that these ones are really the ones who conducted (the killing) and that once in a while law enforcement worked professionally and detained real assassins, and did not make a mistake," Ilya Yashin, the co-chairman of Nemtsov's party, said of the two suspects.

"The key task for investigators is to find and prosecute the ones who ordered this murder. If everything ends with the detention of scapegoats, irrespective of whether they are the real assassins or not, the practice of political assassinations will continue with no doubt."

Yashin and other associates of Nemtsov said that until Saturday they had never heard of the two men detained.

Nemtsov was a liberal who had served as deputy prime minister in the 1990s and later became a staunch critic of Putin. He was shot within sight of the Kremlin walls as he walked home from a cafe.

It was the most high-profile killing of an opposition figure in Putin's 15-year rule.

Putin has called the shooting a shameful tragedy and says he will do everything possible to ensure the perpetrators are found and punished.

Investigators have said the killing could have been a provocation to discredit the Kremlin, linked to Nemtsov's business dealings or his private life, an attack by Islamist militants, or connected to the conflict in Ukraine.

The killing caused shock among Russia's liberal opposition, but they draw their support only from the relatively small urban middle class. The vast majority of Russians back Putin. For them, Nemtsov was a marginal figure tainted by his role in government in the chaotic 1990s.