* Mexican peso crashes to historic low vs U.S. dollar
* S&P 500, Nasdaq 100 futures briefly hit limit down
* Futures down: Dow 3.9 pct, S&P 4.7 pct, Nasdaq 5 pct (Updates prices, changes comment)
By Rodrigo Campos
NEW YORK, Nov 9 (Reuters) - U.S. stock index futures sank in the overnight session into Wednesday as a market that had been expecting a victory by Democrat Hillary Clinton scrambled to adjust to an upset win by Republican Donald Trump.
S&P 500 futures pared losses by more than half as Trump took the stage at his Manhattan headquarters striking a conciliatory tone. He began by thanking Clinton for her many years in public service.
"His victory speech helped turn markets a bit," said Brian Jacobsen, chief portfolio strategist at Wells Fargo Funds Management in Menomonee Falls, Wisconsin.
"He sounded conciliatory and didn't bring up tariffs or the wall, which were probably two of the market's biggest concerns."
Trump campaigned strongly on taxing imports from Mexico and China and building a Wall in the southern border which Mexico would pay for. He also said if he was president Clinton would be in jail.
Through the night, financial markets reacted violently as the results beat the polls and closed Clinton's path to victory. The S&P futures slid 5 percent and hit a limit down, meaning the contract could not trade lower, only sideways or up. Dow Industrials futures briefly fell 800 points.
Republicans were expected to maintain their six-year control over the U.S. House of Representatives, and to defend their Senate majority.
Wall Street is traditionally seen as preferring gridlock, or shared control of the White House and Congress, than a sweep of both Congress houses and the Presidency.
At 3:40 a.m. EST (0840 UTC) S&P 500 e-minis were down 50 points, or 2.34 percent, with 1,822,556 contracts changing hands. Nasdaq 100 e-minis were down 140.5 points, or 2.93 percent, in volume of 226,068 contracts, and Dow e-minis were down 359 points, or 1.96 percent, with 283,436 contracts changing hands.
CBOE Volatility index futures shot nearly 40 percent higher at one point, reflecting investors' reservations over a Trump presidency, but sharply retraced that advance after Trump's acceptance speech. The front-month VIX contract was recently up 12 percent.
"The initial reaction in markets was violent, but gradually we have seen that risk off move retrace quite significantly," said Roger Douglas, a senior portfolio manager at Deutsche Asset Management in London.
"It could be people looking for signs of moderation in his policy, or people starting to wonder whether that moderation will start to come through in the near future."
The Mexican peso slumped versus the U.S. currency to a historic low above 20 per dollar. The peso fell as much as 12 percent versus the greenback and was recently down 7.8 percent at 19.836.
The sharp moves in various financial assets were reminiscent of the reaction to Britain's vote in June to leave the European Union, known as Brexit, which markets misread. S&P e-minis fell 5.7 percent over the two sessions following the vote, but the decline proved a buying opportunity as futures regained their pre-Brexit level within 10 sessions.
"If the selloff in stocks carries through tomorrow and a few days after, it could provide a buying opportunity similar to post-Brexit market action," said Bucky Hellwig, senior vice president at BB&T Wealth Management in Birmingham, Alabama.
While stocks fell, traditional safe havens such as gold and U.S. Treasuries rose as investors avoided risk. The U.S. dollar index fell nearly 2 percent but tumbled 1.8 percent.
Additional reporting by Chuck Mikolajczak, Saqib Ahmed, Caroline Valetkevitch, Sinead Carew, Lewis Krauskopf, Megan Davies and Trevor Hunnicutt in New York, Noel Randewich in San Francisco and John Geddie in London; Editing by Leslie Adler and Louise Heavens