S&P 500 index futures dropped 0.5% after the heated and chaotic debate ended, erasing earlier gains of 0.7%.

With more than a million Americans already casting early ballots, the debates kick off as investors worry about prolonged uncertainty if Trump were to lose, after his refusal to commit to accepting the election outcome.

Here are analysts' reactions:

SHANE OLIVER, HEAD OF INVESTMENT STRATEGY, AMP CAPITAL, SYDNEY

"I didn't see a lot in there on the policy front to change things fundamentally from what was already known.

"The share market normally prefers the incumbent to win. U.S. futures initially rose - as perhaps Trump delivered some punches - but it wasn't enough and I thought Biden did pretty well and so it's no surprise to now see U.S. share market futures down as investors have gone back to worrying about a contested election, a delay in the outcome and whether Trump will go peacefully if he loses."

PHIL BLANCATO, PRESIDENT, LADENBURG THALMANN ASSET MANAGEMENT, NEW YORK

"This was an emotionally charged debate and both sides have strong convictions.

"A Trump victory will lead to a reduction in spending due to the large deficit but no tax hike and less headwinds for the market and an opportunity in value stocks as the economy strengthens.

"The investment implications should Biden win would have to deal with a tax impact at the personal and corporate level which will bring in volatility to the market, but long term could represent an opportunity in healthcare and alternative energy."

JAMES ROSENBERG, FINANCIAL ADVISER, EL&C BAILLIEU, SYDNEY

"Regional investors remain cautious ahead of the election and expect more volatility as rhetoric ramps up. Many believe this is not a good outcome for the U.S."

QUINCY CROSBY, CHIEF MARKET STRATEGIST, PRUDENTIAL FINANCIAL, NEWARK

"If the attempt was to change the minds of anyone undecided I don't think this debate, such as it's called, did the job. It was out of control.

"Trump's space probably enjoyed it tremendously and Biden's space probably were hoping he could get more time to focus on his platform.

"I would have a feeling that it comes out most likely a wash (for the markets.) Although I did find it interesting that the two betting sites that I follow, I'm not saying that they galloped higher for Biden, it took a while, but you did see a little bit of an uptick."

VASU MENON, SENIOR INVESTMENT STRATEGIST, OCBC BANK WEALTH MANAGEMENT, SINGAPORE:

"I think the market outcome from the debate is fairly neutral because it didn't favour either candidate in any big way.

"Wall Street futures only showed a modest uptick and Asian markets showed a mixed performance after the debate probably because the debate did not move the dial in any big way on electorate sentiment."

MASAHIKO LOO, PORTFOLIO MANAGER, ALLIANCEBERNSTEIN, TOKYO

"The markets hadn't had high hopes. The biggest takeaway was that Biden did not turn out to be "Sleepy Joe", and dispelled concerns that he could not cope with debate and would reduce the chance of his winning. So I think the markets pricing of the outcome has not changed much.

"Biden has a lead over Trump but it is not clear if he can get enough electoral colleges, just like in 2016.

"It is becoming more likely that we won't have a clear-cut winner on the night of the election and could have a few weeks of contested period. Some forward-looking markets, such as VIX, are pricing that in but some other markets such as stocks do not seem to have fully factored that in yet. So I expect risk-off trade on worries about a long contested period."

AYAKO SERA, MARKET STRATEGIST, SUMITOMO MITSUI TRUST BANK, TOKYO

"Right now it looks like an even split between Trump and Biden, so it is difficult for the markets to move. What people are most concerned about is the fairness of the election and how it will be carried out.

"There is still a lot of uncertainty, so it is difficult to see a clear trend for the dollar/yen. Under normal circumstances, the positive economic data from China we've seen would support risk-off trades, but this time is different."

(Reporting by Stanley White and Hideyuki Sano in Tokyo, Anshuman Daga in Singapore, Alden Bentley in New York, Scott Murdoch in Hong Kong and Noel Randewich in San Francisco; Compiled by Vidya Ranganathan; Editing by Muralikumar Anantharaman)