TOKYO — Japan's central bank pumped a record $184 billion into money markets and took other measures to protect a teetering economy Monday, as the Tokyo stock market nose-dived following a devastating earthquake and tsunami.

The benchmark Nikkei 225 stock average slid 6.2 per cent in its first day of trading since the 8.9-magnitude quake centred on northeastern Japan struck Friday, triggering enormous waves that swamped towns and killed thousands.

Escalating concerns about the financial and economic fallout -- plus the risk of meltdown at damaged nuclear power reactors -- triggered a plunge that hit all sectors of the stock market. The broader Topix index lost 7.5 per cent.

"Japan will be poorer, for this disaster," said Peter Morici, a business professor at the University of Maryland. "Rebuilding will run down Japan's financial wealth."

The Bank of Japan moved quickly to try to keep financial markets calm. By flooding the banking system with cash, it hopes banks will continue lending money and meet the likely surge in demand for post-earthquake funds.

Later in the day, the central bank's nine-member policy board gathered for a shortened meeting and voted unanimously to ease monetary policy. It will expand the size of an existing program to buy assets -- such as government and corporate bonds -- by 5 trillion yen to 40 trillion yen ($486.4 billion). It also decided to keep its key interest rate at virtually zero.

Credit Suisse economist Hiromichi Shirakawa and analysts at Barclays Capital estimated the damage at up to 15 trillion yen ($183 billion) -- about 3 per cent of gross domestic product. Other experts warned the economy will shrink for two straight quarters.

That represents a painful blow for Japan which lost its place as the world's No. 2 economy to China last year. The Japanese economy has been ailing for two decades, barely managing to eke out weak growth between slowdowns. It is saddled with a massive public debt that, at 200 per cent of GDP, is the biggest among industrialized nations.

"People might see an already weakened Japan, overshadowed by a growing China, getting dealt the finishing blow from this quake," said Koetsu Aizawa, economics professor at Saitama University, north of Tokyo.

Morici said the nuclear crisis combined with the twin hit from the quake and tsunami could make Japan more vulnerable than it was in the past.

"The double whammy has the potential to keep the Japanese economy shut down longer and globalization offers Japan's export customers alternatives they might not have enjoyed a decade or two ago," he said. "Hyundai and Ford now are good substitutes for Toyota's cars, and even more so, Caterpillar tractors made in China can replace Komatsu's land movers."

Among the hardest hit on the stock exchange, shares of The Tokyo Electric Power Co. plunged more than 23 per cent as it faced power shortages and a second hydrogen explosion at a nuclear reactor Monday, sending a massive column of smoke into the air and wounding six workers.

Toyota Motor Corp., the world's biggest automaker, tumbled 7.9 per cent after saying it would suspend manufacturing at its domestic plants through Wednesday -- a production loss of 40,000 cars. Other manufacturers forced to halt production, such as Sony Corp. and Honda Motor Co., also slumped.

The four most severely affected prefectures (states) in the northeast -- Iwate, Miyagi, Fukushima and Ibaraki -- account for about 6 per cent of Japan's economy.

Power supply has failed in the worst affected areas and power rationing may be imposed in other regions. Ports are closed, steel plants have stopped producing, and several major oil refineries have shut down. Getting manufacturing up and working again may be a bigger challenge than in the catastrophic 1995 Kobe earthquake because a larger area is affected.

In the northeastern city of Sendai, the railway station stood deserted. State television footage showed ceilings and walkways collapsed onto the platforms, walls warped and leaning onto the tracks. There was no indication when the station and lines running through could be repaired and operating again.

The northeast is also a major centre for car production, with a myriad of parts suppliers and a network of roads and ports for efficient shipments.

"There is no way to get our products out, even if we make them, with the roads and distribution system damaged," said Honda Motor Co. spokeswoman Natsuno Asanuma.

The aftermath is being felt nationwide.

Four nuclear plants were damaged in the temblors, causing widespread power shortages. Trains in Tokyo, the nation's capital, usually run like clockwork. But are running on a reduced schedule or stopped entirely, preventing millions of commuters from reaching workplaces.

Billions of dollars are expected to be needed to rebuild homes, roads and other infrastructure requiring public spending that will benefit construction companies but add to the national debt.

The economy will eventually get a boost from reconstruction but "this does not mean that Japan is better off," said Julian Jessop, chief international economist at Capital Economics in London. It's a quirk of accounting that destruction of assets is not counted as a reduction in the economy but replacement of those assets boosts economic activity, he said.

Credit Suisse's Shirakawa said in a report the direct economic losses such as property destruction could total 6 trillion yen ($73 billion) to 7 trillion yen. Indirect losses such as lost production will probably be higher.

Other estimates are more pessimistic.

"At the end of the day, this will probably cost a few hundred billion dollars," said Song Seng Wun, economist with CIMB-GK Research in Singapore. "It's going to be a big strain on public finances."

But amid the gloom, Aizawa, the economics professor, says there's reason to be optimistic.

A giant disaster can get Japan to pull together and even provide opportunities for construction and jobs as the recovery gets under way, he said.

"There can be a blessing even in misfortune," he said. "Recovery is about regaining a livelihood for people. No one is going to blame Japan or lower its debt ratings for working on a recovery. This is about lives."