* G20 vows to refrain from competitive devaluations
* Market awaiting further clues on possible Fed easing
* Large option barriers to check euro's rise
* Dlr/yen at 15-yr highs, intervention risks high
(Updates prices, euro options barriers, adds quote)
By Anirban Nag
LONDON, Oct 25 (Reuters) - The dollar dropped broadly on Monday, hitting a 15-year low versus the yen, as a Group of 20 agreement to shun competitive currency devaluations was taken as a green light to resume dollar selling by investors.
At the meeting in South Korea, G20 finance chiefs struck a surprise deal to give emerging nations a bigger voice in the International Monetary Fund, recognising the quickening shift in economic power away from Western industrial nations. They also agreed to exchange rates being market-determined.
Analysts said the outcome pointed to a status quo in currency markets, with the dollar staying under pressure due to market expectations for the Federal Reserve to unveil a second round of quantitative easing as early as November.
"The G-20 was seen as a hurdle by some and now that is over, investors are back to do what they are most comfortable with -- dollar-selling," said Ankita Dudani, G-10 currency strategist at RBS.
The dollar index, which measures its value against a basket of currencies, dropped 0.9 percent to 76.799 with support seen around its 10-month low of 76.14 in the near term. Dudani expected the dollar index to see some choppiness ahead of the Fed meeting on November 2-3.
The dollar fell over 1 percent against the yen to 80.41 yen , its lowest in 15 years. Market players said chances of Japanese yen-selling intervention would increase if the dollar fell below 80.00 yen and tests its record low of 79.75 yen.
The euro climbed 0.75 percent to $1.4053, having broken resistance at $1.4051 earlier in the session. That was a 76.4 percent retracement of the euro's drop to $1.3697 last week from an 8-1/2 month high of $1.4161 hit earlier this month.
With resistance at $1.4051 gone, traders expect $1.4161 peak to be reached soon. But gains above that there were likely to be checked due to the presence of some large option barriers.
Traders cited a one-touch option barrier at $1.4215 that is set to expire on Wednesday. That could lead to a stronger-than-usual defence of that level with the barrier payout said to be a massive 30 million euros. Normal option payouts are usually in the order of 3-5 million euros.
One trader said real money accounts and trend-following commodity trading advisers were seen buying the euro and the Australian dollar, while another cited buying of the euro and the Australian dollar by Asian accounts.
The Australian dollar surged nearly 1.3 percent to $0.9954, getting a boost from news that Singapore Exchange will buy Australian bourse operator ASX.. The Aussie was also helped by expectations of a rate hike early next month.
FOCUS ON FED
That was in sharp contrast to the U.S. where the Federal Reserve looks all set to ease monetary policy further.
While U.S. Treasury Secretary Timothy Geithner reiterated that the United States supports a strong dollar at the G20 meeting, there were few takers for that.
"It is one thing for the Treasury to say that but then the Fed holds all the ammunition and when it is set to print more money, the dollar will remain a weakened currency," said Jane Foley, senior currency strategist at Rabobank.
Analysts at Goldman Sachs said the Fed is almost certain to announce renewed monetary easing at next week's policy meeting. They said it may announce $500 billion in asset purchases or a bit more over a period of about six months, and the size could eventually reach $2 trillion.
In a Reuters poll earlier this month, U.S. primary dealers projected the size of quantitative easing in a range of $500 billion to $1.5 trillion.
German Economy Minister Rainer Bruederle took issue on Saturday with what he called a U.S. policy of increasing liquidity, saying it indirectly manipulated exchange rates.
But such criticism was unlikely to affect the Fed's stance on monetary policy, said a trader for a Japanese trust bank. If it did, it would be tantamount to an admission that the Fed's monetary easing stance up to now had been aimed at weakening the dollar, the trader said. (Additional reporting by Tokyo forex team; editing by Stephen Nisbet)