The Nikkei lost 701.92 points, or 5.28 per cent, to trade at 12,587.4 at 11:30 (0230 GMT) at the end of the morning session.
The broader-based Topix index was down 44.87 points, or 4.09 per cent, at 1,051.67.
Export-linked issues were pulled down by the yen's rise against major currencies. A stronger yen makes Japanese goods less competitive overseas and erodes repatriated revenues.
On currency markets at 11:30 am, the dollar traded at 94.75-78 yen, down sharply from Wednesday's 5 pm quote of 96.80-82 yen.
The euro was quoted at 126.54-58 yen, down from 128.81-85 yen late Wednesday, and at 1.3356-3358 dollars, up from 1.3307-3309 dollars.//DPA