By Sheila Barter BBC News Online

Russian President Vladimir Putin took Russia and the world by surprise by suddenly announcing his entire government had been dismissed.

But the momentum towards the decision had probably been building for months as Mr Putin's allies consolidated their hold on the Kremlin.

Kasyanov was among the last top survivors of the Yeltsin era

Mr Putin's dramatic swipe at his government, less than three weeks before presidential elections, was targeted on one man: his prime minister, Mikhail Kasyanov.

"I would say that it is not the firing of the government. It's the firing of Mikhail Kasyanov," said Sergei Markov of Moscow's Institute of Political Studies.

"According to the constitution, Putin can't do it without firing the whole cabinet."

Wiping out a whole cabinet to remove one man may seem excessive, but Mr Kasyanov represented an entire era, not just a single figure.

It is very positive because it shows that Putin is severing the last links with the Yeltsin era Erik Wigertz United Financial Group

He was a Boris Yeltsin-era appointee, one of the few to have survived the onward march of Mr Putin's allies into the Kremlin.

"It's a symbolic move. Putin stood up and effectively said: I want to make it clear that all ties with Boris Yeltsin's family and its son, Kasyanov, have now been severed," Igor Bunin of the Centre for Political Technologies told Reuters news agency.

Mr Putin's move was also being depicted elsewhere as an "end of empire" move against the Yeltsin men.

"It is very positive because it shows that Putin is severing the last links with the Yeltsin era," said Erik Wigertz of United Financial Group.

Most analysts, Igor Bunin included, had expected the purge to come after the election.

But in the end, psychology as well as symbolism played their part, he believes: Mr Putin simply could not stand Mr Kasyanov, and they were disagreeing on key areas of economic policies.

Family versus clan

Mr Kasyanov was the last big-name survivor of the Yeltsin-era camp at the Kremlin, known as the "family".

The most recent high-profile casualty was Kremlin chief-of-staff Alexander Voloshin, relieved of his duties by the president last October.

Mr Voloshin, like Mr Kasyanov, was a liberal with strong sympathies for the oligarchs who made their fortunes in the chaotic days of privatisation which Mr Yeltsin presided over.

Putin's purge had been expected after the election

As the liberal "family" members have been ousted one by one, their places have been taken by the "clan" members of Mr Putin's entourage.

Some have been allies from Mr Putin's former life as head of the former Russian secret service, the KGB. Others have been from his heartland in St Petersburg.

Mr Voloshin, for example, was replaced by his deputy Dmitry Medvedev, a close Putin associate from St Petersburg.

The loss of Mr Voloshin - a key Kasyanov supporter - left the prime minister exposed. Analysts say his two other claims to the job - his economic management skills and his ties with the oligarchs - have also faded in significance.

'Tsar Vlad'

"The three reasons are no longer important," said Sergei Markov.

The president, admired by many Russians for his strong leadership, is already sailing towards the 14 March presidential elections with an unassailable lead.

His latest tough-man act is expected to earn further electoral respect.

"He wins time - the new government can now be appointed earlier than if he had done this post-election," Mr Wigertz says. "He shows strength and that he is not afraid of controversy."

Mr Putin has already attracted the nickname of "Tsar Vlad" during his steady consolidation on power. His latest decision is likely to strengthen his hand - and his image - still further.