The website of Tokyo-based bitcoin exchange MtGox went down Tuesday amid reports of a theft of the virtual currency worth hundreds of millions of dollars, dealing a blow to its credibility.

The bitcoin community rallied round to defend the unit, with chief executives of several major operators pledging to work together to shore up public faith in the project.

Visitors to the www.mtgox.com domain on Tuesday initially got only a blank page -- consistent with contents having been removed -- more than two weeks after the firm suspended cash withdrawals, claiming there was a bug in the software underpinning the crypto-currency.

Later Tuesday, visitors to the website were greeted with a statement from MtGox: "In the event of recent news reports and the potential repercussions on MtGox's operations and the market, a decision was taken to close all transactions for the time being in order to protect the site and our users. We will be closely monitoring the situation and will react accordingly."

A widely-shared document purporting to be a MtGox "crisis strategy" said the firm might have lost more than 744,400 bitcoins in a theft that had gone unnoticed for years.

That number of bitcoins would be worth more than $300 million, using rates at functioning exchanges Tuesday afternoon.

The figure would represent around five percent of the approximately $5.9 billion global value of bitcoins in existence.

It was not immediately possible to verify the document, which was posted on a blog written by someone who describes himself as "an entrepreneur and former VC who makes the business case for #Bitcoin".

Consternation has grown since MtGox stopped processing external transactions on February 7, claiming there was a problem with the programme that powers the currency and allows it to be transferred between users or swapped for goods and services.

The value of the unit on MtGox had gone into freefall since then. Around midday on Tuesday, shortly before the shutdown, a bitcoin was worth $135. This compares with the $430 quoted by the CoinDesk bitcoin price index, which tracks the price of the currency on major exchanges.

Prices among exchanges are not always the same.

In January a bitcoin was worth more than $900 at MtGox, one of the world's first exchanges for the unit.

- 'Bad actors that need to be weeded out' -

A joint statement issued by CEOs of major platforms said the currency was sound, and blamed MtGox for the fiasco.

"This tragic violation of the trust of users of MtGox was the result of one company's actions and does not reflect the resilience or value of bitcoin and the digital currency industry," said the statement from Coinbase, Kraken, Bitstamp.net, BTC China, Blockchain.info and Circle.

"As with any new industry, there are certain bad actors that need to be weeded out, and that is what we are seeing today. MtGox has confirmed its issues in private discussions with other members of the bitcoin community," it said.

After MtGox's website disappeared, BTC China CEO Bobby Lee told the Wall Street Journal that the Tokyo-based exchange had been on the rocks for more than half a year.

"This has been in the making for a good six to nine months," he told the Journal.

Wild volatility has long been a part of the experimental digital currency, which does not have the backing of a central bank or government and falls outside traditional financial regulatory frameworks.

Units are generated by a complex computer algorithm designed by one or more anonymous people in 2009, with a global cap on the eventual number of bitcoins set at 21 million units.

Proponents say the currency is an efficient and anonymous way to store and transfer monetary value, and to avoid the risks inherent in any currency dependent on the viability of a government for its value.

But some economists say the project is mired in difficulties, including large fluctuations in value caused by speculators and a supposed vulnerability to online thieves.

Others note that the anonymity it offers is attractive to underworld figures and cite its use to buy drugs and guns on the underground Silk Road website.

MtGox has not responded to repeated requests from AFP for comment, but in a statement last week said it was still working on "re-initiating bitcoin withdrawals".

On the eve of the service shutdown, MtGox CEO Mark Karpeles resigned from the board of the Bitcoin Foundation, which is an advocate for the virtual currency.

A handful of MtGox clients have staged small protests outside the firm's headquarters, but say they have not received any reassurances over whether or not their money is safe.