By Rosalind MATHIESON, Colin NG And John PHILLIPS

SINGAPORE—Tokyo shares were down 6.4% in afternoon trading after the massive earthquake and tsunami Friday, but bourses elsewhere in the region were mixed and the Japanese yen fell, a sign global markets were reacting fairly calmly.

But the economic and fiscal damage to Japan from the 8.9-magnitude quake remained unclear, while an emergency continued at Tokyo Electric Power Co.’s Daiichi nuclear-power plant in the country’s northeast, with an explosion in its No. 3 reactor building.

Investors were also looking at the ripple effects from the weekend euro-zone debt accord and the continuing Libya crisis, particularly the Arab League’s support for a no-fly zone over the country. In Asia, the euro-zone news was taken as positive for the euro, which gained against the U.S. dollar and the yen.

In initial Asia trading, the yen rose to an early four-month high against the U.S. dollar but then slipped back, partly as a result of the Bank of Japan’s offer to inject a huge amount of cash into the short-term money market, aimed at securing market confidence.

The benchmark Nikkei Stock Average, having broken below the key 10,000 level, was recently at 9599.11. Australia’s S&P/ASX 200 was off 0.4% and South Korea’s Kospi Composite was up 0.3%. Hong Kong’s Hang Seng Index was 0.5% lower, Taiwan’s main index had shed 0.6%, and the Shanghai Composite index was down 0.3%.

Dow Jones Industrial Average futures were down 49 points in screen trade.

All eyes are on Monday morning’s markets as Japan scrambles to avoid a nuclear disaster. Plus: how will the quake impact insurance companies, manufacturers, and the global supply chain? Simon Constable and John Bussey discuss with Dow Jones Newswires’ Michael Casey.

Investors’ nerves have already been tested recently by renewed concerns over European sovereign debt, the escalating conflict in Libya and political protest in Saudi Arabia, along with data signaling a Chinese economic slowdown.

“We possibly cannot ignore the impact that this quake will have in terms of geographical span and scale—as well as the psychological impact,” said Credit Suisse strategist Shun Maruyama in Tokyo.

Still, some analysts were tipping Japan stock declines to slow. Nicholas Smith, director for equity research at MF Global FXA Securities, said while the quake was horrific, “in total, the area affected by the quake and tsunami does not comprise a big-enough chunk of Japan’s gross domestic product to warrant an over reaction (in stock selling) based on that criteria alone.”

Shares of Tokyo Electric Power—operator of the damaged power plant—were ask-only, meaning the gap between buyers’ bids and sellers’ offers was so wide no trades were being done. Nissan Motor was down 11%, Honda Motor was down 7.9% and Toyota Motor shed 8.6% after they said they will suspend operations at nearly all their domestic plants. Financial shares were sharply lower, with Sumitomo Mitsui FG down 6.9%.

Oil companies JX Holdings and Cosmo Oil, which had refinery fires following the earthquake, were down 16% and 25%, respectively.

Trading in Chinese shares was choppy as market participants weighed the implications of the Japanese disaster, while lower-than-expected data on new yuan loans for February indicated that Beijing’s control on credit is tighter than anticipated.

Elsewhere, Australian steelmakers and thermal coal miners were outperformers on the theory that thermal coal could be a major beneficiary of the shutdown of parts of Japan’s nuclear-power network, while uranium companies fell as the problems in Japan stoked concerns about nuclear power’s longer-term viability around the world.

Travel-related shares in Korea, such as airline stocks, were sharply lower on concerns about a decline in the number of tourists from Japan, while heavy-industry stocks also posted steep declines due to concerns that the Japan quake may lead to higher steel prices.

Shares in Taiwan were weighed by losses in tech shares on concerns that Japan’s earthquake was likely to hurt electronics product shipments, though DRAM-memory-chip makers as well as petrochemical, steel, and cement stocks were stronger as they were the most likely to benefit from Japan-related supply shortages.

Elsewhere in the region, Malaysian shares were flat, Singapore’s Straits Times Index was off 0.5%, New Zealand’s NZX-50 was 0.6% lower, Indonesia’s share market was up 0.7%, Thailand shares were 0.7% higher, while India’s Sensex was up 0.7%.

In foreign exchange markets, the U.S. dollar was higher at around 82.12 yen, after touching a four-month low of 80.60 yen earlier in Asia. That compared with 81.83 yen in late New York trade on Friday.

The dollar/yen pair was helped off lows by news that the Bank of Japan injected 18 trillion yen of funds into the market—15 trillion yen in same-day funds through three separate operations plus three trillion yen in repurchase agreements—to help ensure smooth operations in the financial system.

The euro was at $1.3927, from $1.3901 in late New York, and at 114.34 yen, from 113.82 yen.

Gains for the euro came after European leaders agreed at a summit in Brussels to extend the effective lending capacity of the European Financial Stability Facility, the special vehicle set up last May to help out euro-using states abandoned by the markets.

However, they stopped short of allowing the stability facility to buy government debt in the secondary markets. The European Central Bank had hoped the facility would take off its hands some of the 77.5 billion euros, or about $108 billion, in government bonds it has bought over the past 10 months to prevent the seizing-up of debt markets in Greece, Ireland and Portugal.

Lead Japanese government bond futures surged 0.60 to 139.80 points as stocks in Tokyo fell, while the 10-year cash JGB yield was down 0.03 percentage point at 1.215%.

Oil and commodity markets were being driven largely by Japanese headlines.

April Nymex crude-oil futures were down $1.65 at $99.51 per barrel on Globex.

The three-month London Metals Exchange copper contract was down $79 at $9,111 per ton, reversing an earlier rise. Copper already declined last week on concerns about the level of Japanese demand and as risk appetite turned sour.

Spot gold rose on a flight-to-safety bid and was at $1,421.90 per troy ounce, up $2.30 from its New York settlement Friday.

—David Cottle in London, Joe Bel Bruno in New York and David Fickling in Sydney contributed to this article.

Write to Rosalind Mathieson at rosalind.mathieson@dowjones.com