Kremlin challenger Boris Nadezhdin says he has collected enough signatures to stand as a candidate in Russia's upcoming presidential election.

The former MP has become known for his relatively outspoken criticism of Mr Putin and of Russia's full-scale invasion of Ukraine.

Mr Nadezhdin said he handed over more than the 100,000 required signatures to the electoral authorities.

The electoral commission must now review his application.

If any "irregularities" are found with any of the submitted signatures, the commission can disqualify the candidate altogether. Independent politician Yekaterina Duntsova was disqualified from running in December when the electoral commission said it had found 100 "mistakes" on her form.

Current President Vladimir Putin has already registered as an independent candidate for the election in March, which will almost certainly see him win another six-year term.

Shortly after today's deadline to hand in the signatures, Mr Nadezhdin posted a photo of himself standing in front of several boxes containing papers bearing the signatures of his supporters.

"This is my pride - the work of thousands of people over many sleepless days. The result of the queues you stood in in the freezing cold is in those boxes," he wrote on X, formerly Twitter.

Thousands of Russians have queued in the cold across the country to add their signature to the list of people supporting his bid.

Mr Nadezhdin, 60, a member of the State Duma from 1999-2003, was nominated to stand in the election by the centre-right Civic Initiative party in December 2023.

He has long been a frequent guest on talk shows on state TV channels, where he has often criticised Russia's war on Ukraine.

He recently said the president had "practically destroyed the key institutions of the modern state of Russia", and told the BBC that, if elected, his first task would be to end the war.

In a country where opposition figures have been jailed or even assassinated, his recent criticism of Mr Putin appears to have been tolerated so far.

When asked about Mr Nadezhdin earlier this month, Kremlin spokesman Dmitry Peskov said: "We don't see him as a rival."

On Tuesday, Mr Nadezhdin told the BBC's Newshour programme that he had "no fear" about running, although he also said elections in Russia "are not free and not fair".

Asked about his criticism of the war in Ukraine, he said that invading the country was a "fatal mistake" which "dragged [Russia] into the track of authoritarianism and isolation".

"First of all we should finish killing each other and then there will be long, long negotiations about the border," he said.

Even if Mr Nadezhdin is allowed to run, it is not yet known if he will be allowed to campaign freely.

In Vladimir Putin's Russia, candidates have previously been able to run in elections when it is generally assumed that they don't stand a chance, and sometimes they don't even campaign against the incumbent.

This maintains a facade of democracy, and in the case of Mr Nadezhdin it would allow Russians dissatisfied with the "special military operation" to vent their anger and frustration in a way that would not threaten Mr Putin's rule.

In recent years, genuine popular opposition figures - such as Alexei Navalny and Ilya Yashin - have been handed lengthy jail sentences. Others, such as Kremlin critic Boris Nemtsov - for whom Mr Nadezhdin once worked as an adviser - have been killed.

Russia's political scene has been dominated by Vladimir Putin since 2000. In 2020, a constitutional amendment was passed allowing him to stay in power beyond 2024.

A victory in March would see him remain as president until 2030. After that, he can then potentially serve another six years until 2036 if he decides to stand again.