Tokyo stocks received a much anticipated pummeling Monday, with the key Nikkei stock index plunging 6.18 percent to a four-month closing low as factories, production lines and ports ground to a halt over the weekend following Friday's megaquake.

A man walks past a stock price board in a street, March 14, 2011 in Tokyo, Japan. (Xinhua/AFP Photo)

Unsurprisingly, the construction sector was the only one that posted gains Monday and brokers said that insurance firms led the decliners, followed by resource-related issues and consumer lenders.

The impact of Friday's 9.0 magnitude quake that leveled a number of towns and villages up and down the northeastern Pacific coast of Japan, was further exacerbated by a series of devastating tsunamis which claimed the majority of the 10,000 people now feared dead.

The Nikkei hit its lowest intraday level since November 5, following reports of a stricken nuclear reactor exploding.

Officials said that any radioactive substances that had leaked into the air as a result of the explosion would not be harmful to human's health.

Monday's nuke plant crisis follows a similar explosion of a reactor building on Saturday, as workers frantically tried to ease growing pressure building in the plants and cool their melting fuel rods.

In both cases, the reactor and the containment vessel were declared safe and undamaged following the explosions by the plant' s owner and operator, Tokyo Electric Power Co. (TEPCO) and verified by the central government's nuclear agency.

"The size of the economy of the main earthquake-affected region is roughly the same as that of the area hit by the Great Hanshin ( Kobe) earthquake in 1995, but with this Tohoku Pacific (Sendai) earthquake affecting road networks, power plants and other infrastructure over a wide area, we expect the short-term economic impact to be greater than the Kobe earthquake," Nomura Holdings Inc. economists said in a report on the matter.

The 225-issue Nikkei Stock Average plummeted 633.94 points from Friday to 9,620.49, to book its biggest one day loss since a month after the collapse of Lehman Brothers Holdings Inc.

The broader Topix index of all First Section issues on the Tokyo Stock Exchange, meanwhile, fell 68.55 points, or 7.49 percent, to close the first trading day of the week at 846.96.

But the Nikkei came back from its intraday low following the Bank of Japan announcing additional monetary easing measures including an expansion of the asset-buying program it introduced in October. The BOJ made the announcement just before the markets closed.

The BOJ decided to expand the size of its asset-buying scheme by 5 trillion yen to 10 trillion yen (121 billion U.S. dollars) at its policy meeting today while keeping interest rates unchanged. Japanese financial markets, according to brokers, were largely unmoved by the central bank's decision.

Owner and operator of the trouble nuclear power plant in Fukushima Prefecture, Tokyo Electric Power, plummeted 23.6 percent to 1,621 yen after staying bid-only for the majority of today's trading hours.

Tokyo Electric Power will start planned power outages in parts of the greater Tokyo area Monday evening, the firm said, having checked the supply and demand situation.

The biggest supplier of electricity in the capital will divide its service area into five groups, each with three-hour outages. Two outages planned for this morning were suspended as Tokyo Electric said supply at that time was ample.

Construction-related businesses rallied on the back of expectations for demand from rebuilding efforts, with Kajima soaring 22.2 percent to 259 yen and Taiheiyo Cement jumping 21.2 per cent to 137 yen.

According to Citi research, in the month following the Kobe earthquake in Japan 1995, construction shares surged and went on to outperform the broader market in the month after the earthquake consistently for an entire year.

Japanese automakers lost ground on Monday as many have them have a shortage of supplies and Toyota Motor slumped 7.9 percent to 3,310 and Nissan Motor skidded down 9.5 percent to 722 yen. Meanwhile, Honda lost 6.5 percent to 3,095 yen.

Among electronics makers, Sony dropped 9.1 percent to 2,550 yen after the company halted production at some of its factories and Toshiba plunged 16 percent to 411 yen and Hitachi also fell 16 percent to 414 yen.

Insurance shares were a heavy weigh Monday, with Tokio Marine Holdings down 12 percent at 2,200 yen and Dai-ichi Life Insurance plummeting 19 percent to 118,700 yen on concerns about the level of insurance payout claims in the days and weeks following Friday' s devastation.

Strategists have pointed out however that the Nikkei's fall comes after several months of solid performance, in which Tokyo shares outperformed their emerging Asian peers, and overseas investors flocked to Japanese equities, buying roughly 3 trillion yen's worth since November.

As of Thursday, the day before the earthquake struck, the Nikkei had climbed 13 percent in the five months starting in November, far outpacing a 1.7 percent gain in MSCI's index of Asia- Pacific shares outside Japan.

But analysts are saying that the market could test its downside in the coming weeks as the destruction and impact on corporate activity in Japan unfold further.

Trading volume on Monday leapt to 4.88 billion shares on the Tokyo Exchange's First Section, up from Friday's volume of 3.14 billion shares, with declining stocks outnumbering advancing ones by 1,571 to 103.

VietNamNet/Xinhuanet