MOSCOW, June 24 (Reuters) - Russia accused mercenary chief Yevgeny Prigozhin of armed mutiny on Friday after he alleged, without providing evidence, that the military leadership had killed a huge number of his fighters in an air strike and vowed to punish them.

The standoff, many of whose details remained 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.

As the standoff between Prigozhin, founder of the Wagner mercenary force, and the defence ministry appeared to come to a head, the ministry issued a statement saying Prigozhin's accusations were "not true and are an informational provocation".

Prigozhin said his actions were not a military coup. But in a frenzied series of audio messages, in which the sound of his voice sometimes varied and could not be independently verified, he appeared to suggest that 25,000 fighters were en route to oust the leaders of the defence establishment in Moscow.

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

At about 2 a.m. on Saturday morning, Moscow time (2300 GMT), Prigozhin issued a new message saying his forces had crossed the border from Ukraine, and were in the southern Russian city of Rostov.

He said they were ready to "go all the way" against the top brass, and to destroy anyone who stood in their way.

At around the same time, the state news agency TASS quoted Kremlin spokesman Dmitry Peskov as saying all Russia's main security services were reporting to Putin "round the clock" on the fulfilment of his orders with respect to Prigozhin.

Security was being tightened in Moscow, TASS said, focusing on what it called the capital's most important government sites and infrastructure.

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

The FSB domestic security service said it had opened a criminal case against him for calling for an armed mutiny, a crime punishable with a jail term of up to 20 years.