TOKYO (AFP) - Tokyo stocks opened 0.75 per cent lower on Friday after a scare over a Portuguese bank sent Wall Street lower and boosted the yen.

The Nikkei 225 index was down 113.88 points at 15,102.59 at the start.

"Investors have gone more 'risk-off' in light of how fully valued many global stock markets have become and how vulnerable they are to potential sell-offs," said SMBC Nikko Securities general manager of equities Hiroichi Nishi.

The yen, often considered a safe haven in financial turmoil, rose as the Portuguese bank scare spread.

The Japanese currency fetched 101.28 to the dollar and 137.76 to the euro early Friday, compared with 101.32 and 137.88 in New York Thursday afternoon.

The dollar bought US$1.3601 against US$1.3609 in US trade.

A strong yen is negative for Japanese exporters as it makes them less competitive abroad and erodes profits when repatriated.

The Dow Jones Industrial Average on Thursday finished down 0.42 per cent at 16,915.07 after recouping some of a sharp opening loss.

Sell-offs hit European markets Thursday due to fears over the health of Portugal's largest listed bank, Banco Espirito Santo (BES).

Portugal's financial market regulator halted trading in BES shares after they plunged over allegations that its parent company covered up a 1.3-billion-euro (S$2.2 billion) hole in the accounts.