By Andrew Osborn and Kevin Liffey

LONDON (Reuters) -Rebellious Russian mercenary chief Yevgeny Prigozhin said on Saturday he had taken control of the Russian city of Rostov-on-Don as part of an attempt to oust the military leadership amid what the authorities said was an armed mutiny.

Prigozhin demanded that Russian Defence Minister Sergei Shoigu and Valery Gerasimov, the chief of the General Staff, whom he has pledged to oust over what he says is their disastrous leadership of the war against Ukraine, come to see him in Rostov, a city near the Ukrainian border.

He had earlier said that he had 25,000 fighters moving towards Moscow to "restore justice" and had alleged, without providing evidence, that the military had killed a huge number of fighters from his Wagner private militia in an air strike, something the defence ministry denied.

"Those who destroyed our lads, who destroyed the lives of many tens of thousands of Russian soldiers, will be punished. I ask that no one offer resistance...," he said in one of many frenzied audio messages.

"There are 25,000 of us and we are going to figure out why chaos is happening in the country," he said, promising to destroy any checkpoints or air forces that got in Wagner's way.

Prigozhin, whose Wagner militia spearheaded the capture of the Ukrainian city of Bakhmut last month, has for months been openly accusing Shoigu and Gerasimov of incompetence and of denying Wagner ammunition and support in its battles in Ukraine.

The dramatic turn, with many details unclear, looked like the biggest domestic crisis President Vladimir Putin has faced since he ordered a full-scale invasion of Ukraine - something he called a "special military operation" - in February last year.

Putin was due to address the nation soon, the RIA news agency cited the Kremlin as saying on Saturday.

Russia's FSB security had earlier opened a criminal case against Prigozhin for armed mutiny and had said that his statements were "calls for the start of an armed civil conflict on Russian territory and his actions a 'stab in the back' of Russian servicemen fighting pro-fascist Ukrainian forces".

It added: "We urge the ... fighters not to make irreparable mistakes, to stop any forcible actions against the Russian people, not to carry out the criminal and traitorous orders of Prigozhin, to take measures to detain him."

The state news agency TASS quoted Kremlin spokesman Dmitry Peskov as saying that all of Russia's main security services were reporting to Putin "round the clock."

Security was being tightened in Moscow, Mayor Sergei Sobyanin said on his Telegram channel.

In Washington, U.S. President Joe Biden was briefed on the situation, a White House spokesperson said.

On Friday, Prigozhin had appeared to cross a new line in his increasingly vitriolic feud with the ministry, saying that Putin's stated rationale for invading Ukraine 16 months ago was based on lies concocted by the army's top brass.

"The war was needed ... so that Shoigu could become a marshal ... so that he could get a second 'Hero' [of Russia] medal," Prigozhin said in a video clip.

"The war wasn't needed to demilitarise or denazify Ukraine," he said, referring to Putin's justifications for the war.

About 2 a.m. (2300 GMT), Prigozhin posted a message on the Telegram app saying his forces were in Rostov and ready to "go all the way" against the top brass and destroy anyone who stood in their way.

About 5 a.m. (0200 GMT), the administration of the Voronezh region, on the M-4 motorway between the regional capital Rostov-on-Don and Moscow, said on Telegram that a military convoy was on the highway and urged residents to avoid using it.

Unverified footage posted on social media showed a convoy of assorted military vehicles, including at least one tank and one armoured vehicle on flatbed trucks. It was not clear where they were, or whether the covered trucks in the convoy contained fighters. Some of the vehicles were flying the Russian flag.

Footage on channels based in Rostov-on-Don showed armed men in military uniform skirting the regional police headquarters in the city on foot, as well as tanks positioned outside the headquarters of the Southern Military District.

Reuters confirmed the locations shown but could not determine when the footage was shot.

MILITARY CONVOY

Prigozhin denied that he was trying to stage a military coup.

He said he had led his fighters out of Ukraine to Rostov, where a video posted by a pro-Wagner Telegram channel showed him, seemingly relaxed, conversing with two generals at the headquarters of Russia's huge Southern Military District.

The video showed him telling the generals: "We have arrived here, we want to receive the chief of the general staff and Shoigu. Unless they come, we'll be here, we'll blockade the city of Rostov and head for Moscow."

That appeared to be a reference to a military convoy trying to make a 1,200-km (750-mile) drive towards Moscow, ostensibly to topple the military leadership.

Russian local officials said a military convoy was indeed on the main motorway linking the southern part of European Russia with Moscow, and warned residents to avoid it.

Army Lieutenant-General Vladimir Alekseyev - who was later to appear with Prigozhin in the video from Rostov-on-Don - issued a video appeal asking Prigozhin to reconsider his actions.

"Only the president has the right to appoint the top leadership of the armed forces, and you are trying to encroach on his authority," he said.

Army General Sergei Surovikin, the deputy commander of Russian forces in Ukraine, whom Prigozhin has praised in the past, said in a video that "the enemy is just waiting for our internal political situation to deteriorate".

"Before it is too late ... you must submit to the will and order of the people's president of the Russian Federation. Stop the columns and return them to their permanent bases," he said.

An unverified video on a Telegram channel close to Wagner showed the purported scene of an air strike against Wagner forces. It showed a forest where small fires were burning and trees appeared to have been broken by force. There appeared to be one body, but no more direct evidence of any attack.

It carried the caption: "A missile attack was launched on the camps of PMC (Private Military Company) Wagner. Many victims. According to eyewitnesses, the strike was delivered from the rear, that is, it was delivered by the military of the Russian Ministry of Defence."

(Reporting by Reuters; Writing by Andrew Osborn and Kevin Liffey; Editing by Daniel Wallis and William Mallard)