ROSTOV-ON-DON/VORONEZH, Russia, June 24 (Reuters) - Russian military helicopters opened fire on Saturday afternoon on a convoy of rebel mercenaries already more than half way towards Moscow in a lightning advance after seizing a southern city overnight.

President Vladimir Putin vowed to crush an armed mutiny he compared to Russia's Civil War a century ago.

Fighters from Yevgeny Prigozhin's private Wagner militia were in control of Rostov-on-Don, a city of more than a million people close to the border with Ukraine, and were rapidly advancing northwards through western Russia.

A Reuters journalist saw army helicopters open fire at an armed Wagner column that was advancing past the city of Voronezh with troop carriers and at least one tank on a flatbed truck. The city is more than half way along the 1,100-km (680-mile) highway from Rostov to Moscow.

Prigozhin, whose private army fought the bloodiest battles in Ukraine even as he feuded for months with the top brass, said he had captured the headquarters of Russia's Southern Military District in Rostov after leading his forces into Russia from Ukraine.

In Rostov, which serves as the main rear logistical hub for Russia's entire invasion force, residents milled about, filming on mobile phones, as Wagner fighters in armoured vehicles and battle tanks took up positions.

One tank was wedged between stucco buildings with posters advertising the circus. Another had "Siberia" daubed in red paint across the front, a clear statement of intent to sweep across the breadth of Russia.

In Moscow, there was an increased security presence on the streets. Red Square was blocked off by metal barriers.

"Excessive ambitions and vested interests have led to treason," Putin said in a televised address, comparing the insurrection at a time of war abroad to Russia's revolution and civil war unleashed during World War One.

"All those who deliberately stepped on the path of betrayal, who prepared an armed insurrection, who took the path of blackmail and terrorist methods, will suffer inevitable punishment, will answer both to the law and to our people."

A defiant Prigozhin swiftly replied that he and his men had no intention of turning themselves in.

"The president makes a deep mistake when he talks about treason. We are patriots of our motherland, we fought and are fighting for it," Prigozhin said in an audio message. "We don't want the country to continue to live in corruption and deceit."

In a series of hectic messages overnight, Prigozhin demanded Defence Minister Sergei Shoigu and the chief of the general staff Valery Gerasimov come to see him in Rostov.

Western capitals said they were closely following the situation in nuclear-armed Russia. The White House said President Joe Biden was briefed.