* S&P 500 futures open higher after polls close
* Opinion poll puts "remain" campaign in lead
* Twilio Inc nearly doubles in welcome return of tech IPOs
* Indexes up: Dow 1.3 pct, S&P 1.3 pct, Nasdaq 1.6 pct (Updates to after close of polls in UK)
By Rodrigo Campos
June 23 (Reuters) - U.S. stock index futures opened higher in after hours trading on Thursday after polls closed in Britain with early signs that the "remain" campaign is leading in the country's EU membership referendum.
The gains added to a rally in stocks earlier on Wall Street as investors bet the United Kingdom would remain part of the European Union, potentially avoiding damage to European trand its consequences for the global economy.
The campaign to keep Britain in the European Union appeared to be ahead, according to polling firm YouGov which surveyed people about how they voted. The final official result is expected around 0600 GMT.
S&P 500 E-mini futures rose 0.5 percent and E-mini Dow Jones futures rose 0.52 percent.
Sterling rose more than a cent to a fresh 2016 high of $1.5004 from $1.4975 before polling stations across the country closed.
"People are expecting that Britain is going to vote to stay in the EU," said Peter Jankovskis, co-chief investment officer at OakBrook Investments LLC in Lisle, Illinois.
"They view the potential disruption of a British exit as a major financial event," he said.
Earlier, the Dow Jones Industrial Average rose 230.24 points, or 1.29 percent, to 18,011.07, the S&P 500 gained 27.87 points, or 1.34 percent, to 2,113.32 and the Nasdaq Composite added 76.72 points, or 1.59 percent, to 4,910.04.
The Dow Industrial Average closed back above 18,000 and at its highest since April 27. The financial sector led the S&P 500 stock index with a 2.1 percent gain.
The CBOE Volatility index fell 18.5 percent, its largest daily percentage decline since October 2013.
The S&P 500 index closed 0.8 percent below its record closing high hit May last year.
Software maker Twilio Inc nearly doubled in its market debut, rising as high as $29.60 after pricing at $15. It closed up 91.9 percent at $28.79.
About 6.4 billion shares changed hands in U.S. exchanges, below the 6.8 billion average over the past 20 sessions.
Advancing issues outnumbered declining ones on the NYSE by a ratio of 4.98-to-1 and on the Nasdaq, a 3.57-to-1 ratio favored advancers.
The S&P 500 posted 52 new 52-week highs and 2 new lows; the Nasdaq recorded 88 new highs and 26 new lows.
Reporting by Rodrigo Campos; editing by Clive McKeef