* Nikkei tumbles more than 6 pct, European shares fall 1 pct * Market falls begin to level off ahead of Wall Street open * Dollar slides to 10-week low vs yen, 3-1/2 month trough vs euro * Persistent Fed stimulus doubts stirs liquidations by funds By Marc Jones LONDON, June 13 (Reuters) - World shares fell and the dollar slumped on Thursday as a sell-off on global financial markets accelerated on concerns over whether central banks will continue the stimulus they have come to rely on. Stocks, bonds, commodities and the dollar were all caught in the selling. There were tentative signs markets were levelling off ahead of U.S. trading, although Wall Street was also expected to open in the red. The slide has been triggered by comments from policymakers at the U.S. Federal Reserve, which meets next Tuesday and Wednesday, about when to start scaling back its huge bond-buying programme. European shares were down 1 percent ahead of the U.S. restart, after Japan's Nikkei fell 6.4 percent - its second-biggest daily drop in more than two years - rattling markets and leaving Asian shares at their lowest level in 2013. Heavy selling hit the dollar, which slumped as much as 2 percent against the yen as investors spooked by Japan's stock dive unwound bets the yen would weaken. It fell as low as 93.90 yen, its lowest since April 4, giving back almost all the gains made since the Bank of Japan's aggressive monetary easing announced on that day. "If you look at it historically, there has never been a period when the Fed has started to take back stimulus that has left the markets untouched," said Hans Peterson, global head of investment strategy at Swedish bank SEB. "And this time it is a bigger exercise. We have moved markets from 2009 to 2013 on stimulus and now we are trying to take a step into a world which is more driven by natural growth. That transition will not be easy." TOKYO TANKS Emerging market assets have been particularly hit over the last few weeks as the uncertainty about central bank stimulus has driven a global dash back to cash and core economies. They were hit again as the European afternoon session continued. Emerging equities fell to 11-month lows and most emerging currencies remained under heavy pressure with the Indian rupee falling to a record low. In the debt market, German government bonds saw their biggest gains in a week and U.S. treasuries made ground as investors headed for traditional safe-haven paper. The recent selling of euro zone periphery debt also resumed , and Italy saw its borrowing costs rise at an auction of three-year debt although yields at a parallel 15-year sale were little changed. The rout in Asian markets saw many of them plummet to multi-month lows as investors scrambled to recalibrate positions for a world with potentially reduced liquidity support. Both the dollar/yen and the Nikkei fell below the Ichimoku cloud bottom for the first time since their rallies began in November, sending a strong bear market signal. The 6.4 percent fall in Tokyo's Nikkei also breached its 50 percent retracement from its November rise and brought its losses over the last two weeks or so to more than 20 percent. TRICKY TRANSITION Among U.S. economic reports due later will be retail sales, import prices, business inventories and weekly jobless claims, all likely to feed into calculations of when the Fed will start phasing out its support ahead of its meeting next week. "The trend is still in principle a sell-off in markets, a sell-off in riskier assets on the expectations that the Fed might signal further readiness to maybe slow down the rate of purchases," said Daiwa Securities economist Tobias Blattner. "So all eyes are on the FOMC meeting next week. There is very little else that matters at the moment" With U.S. stocks looking set for early falls, the dollar remained near to a 3-1/2 month low against the euro as a slight rebound by the greenback against a basket of major currencies left the common currency buying $1.3320. Gold was also pinned back by Fed stimulus caution. But with oil bang in the middle of its recent $100-105 range and copper only slightly lower, commodity markets were largely spared the drama unfolding elsewhere.