* FTSE 100 up 0.5 pct; U.S. Fed renews zero rate pledge
* Eyes on Scottish referendum vote; unionists just ahead
* TUI Travel boosted by Morgan Stanley upgrade
* Monitise tumbles as Visa explores options for stake
By Tricia Wright
LONDON, Sept 18 (Reuters) - Britain's top shares rose on Thursday as investors put aside angst about an independence vote that got under way in Scotland to focus on the prospect of an extended period of ultra-low interest rates in the United States.
Equities across Europe temporarily trimmed gains when the European Central Bank handed out a below-forecast 82.6 billion euros ($106.38 billion) in its first offering of four-year loans to banks.
But traders said this was offset by expectations that a small figure might prompt the ECB to take more monetary stimulus measures, such as buying assets to print money - a tool known as quantitative easing (QE).
The FTSE 100 was up 31.37 points, or 0.5 percent, at 6,812.27 points by 1505 GMT, propped up by the U.S. Federal Reserve's reiteration on Wednesday of its pledge to keep ultra-low interest rates for a "considerable time".
Scots started voting at 0600 GMT on Thursday. Five surveys showed support for independence at 48 percent, compared with 52 percent for the union, while a fifth poll showed it even closer at 49 percent to 51 percent. Another showed unionists at 53 percent and separatists at 47 percent.
Investors reckoned that markets were starting to price in a win for those in favour of remaining in the United Kingdom, and saw scope for a sharp rally in the event of such a result.
"The polling stations in Scotland may be open for a good few hours yet, but the City appears to have already concluded that this is a win for the 'No' campaign," Tony Cross, market analyst at Trustnet Direct, said.
"I think a 'No' will send stocks much higher tomorrow."
TUI Travel jumped 4.5 percent to the top of the blue-chip leader board after Morgan Stanley upgraded its rating on the stock to "overweight" from "equal-weight" following the tour operator's recent merger agreement with majority owner TUI AG.
Volume in TUI Travel stood at almost three times its 90-day daily average volume, against that for the FTSE 100 at just three quarters.
Among other notable movers, small-cap Monitise tumbled more than 30 percent after Visa Inc said it was exploring options for its 5.5 percent stake in the British mobile banking technology company. Its trading volume was seven times its 90-day daily average. (1 US dollar = 0.7765 euro) (Additional reporting by Sudip Kar-Gupta; Editing by Hugh Lawson)