A man watches a stock price board on a Tokyo street on Monday. (Eugene Hoshiko/Associated Press)

The Tokyo Stock Exchange plunged on Monday as investors reacted to the consequences of a massive natural disaster that is still unfolding.

Japan's central bank injected 15 trillion yen ($184 billion US) into money markets Monday to stem worries about the world's third-largest economy.

The benchmark Nikkei 225 stock average shed nearly 634 points, or 6.2 per cent, to 9,620.49, extending losses from Friday. Escalating concerns over the fallout of the disaster triggered a plunge that hit all sectors. The broader Topix index lost 7.5 per cent.

Japan's economy has been ailing for 20 years, barely managing to eke out weak growth between slowdowns. It is saddled by a massive public debt that, at 200 per cent of gross domestic product, is the biggest among industrialized nations.

Other Asian markets were off but just a fraction of the losses in Tokyo. The Hang Seng index in Hong Kong fell and Singapore's Straits Times index were both down about 0.5 per cent.

The tsunami and earthquake that hit Japan on Friday about the time the market closed may have killed more than 10,000 people. It has caused a massive disruption to transportation in the northeastern part of the country, and led to severe damage to several nuclear plants.

The electricity shortage caused by the damage has forced leading Japanese car companies — Toyota, Honda and Nissan — to close for an indefinite period, and electricity companies are warning of rotating blackouts on Monday to conserve power.

However, the Tokyo exchange said Sunday that it would keep its normal hours on Monday.

On Monday morning, the Central Bank conducted a same-day funds-supplying operation totalling seven trillion yen, and a future-day-start funds-supplying operation totalling three trillion yen. The bank said it will do its utmost to continue ensuring stability in the financial markets and securing smooth settlement of funds, including providing liquidity.

On Friday, the Nikkei fell 200 points to about 10,254.

The 1995 earthquake that devastated Kobe cost $132 billion US, said Sheila Smith, an expert in Japan Studies at the Council on Foreign Relations, a U.S. think tank.

With files from The Associated Press