The sound of ambulance and police sirens filled the air on Saturday in Rostov-on-Don -- a hub for Russia's Ukraine campaign -- where the rebellious Wagner mercenary force said it had taken over key facilities.

At a major intersection in the city centre, an armoured car with a machine gun and around a dozen men in military fatigues with silver armbands could be seen.

Armoured personnel carriers and tanks were positioned in other parts of the centre, including outside a toy shop and a circus, journalists in the city reported.

Passers-by stopped to look at the military vehicles, including transport trucks, and more armed men with silver armbands carrying rifles in resting positions.

Wagner chief Yevgeny Prigozhin has said his troops had taken control of Russia’s main military command centre for Ukraine operations as well as an airbase in the city, vowing to topple Moscow's top military leaders.

He said he commanded around 25,000 fighters.

A port city located just over 100 kilometres from the Ukrainian border, Rostov-on-Don was founded in the 18th century and is the headquarters of Russia's southern military command.

Unlike other large Russian cities further from the front lines, Rostov-on-Don -- with a population of just over one million people -- has experienced the ripple effects of Russia's large-scale military operation in Ukraine.

One person was killed in March at an FSB building fire in Rostov-on-Don and the Rostov region has also been targeted over recent months in several drone attacks.

Regional governor Vasily Golubev called on residents not to leave their homes unless absolutely necessary and said any planned mass gatherings in the city had been cancelled.

He also asked the public not to use the M4 motorway -- the main road connection between Moscow and southern Russia -- as sections have been closed and large queues of cars have formed along it.

Golubev said public transport in the city was still functioning but routes had been changed to avoid the city centre.

"The situation that has arisen requires a maximum concentration of efforts to keep order," he said on social media.

"Law enforcement agencies are doing everything necessary to ensure the security of our region's residents."

bur/giv