So let's be honest.  There was really no reason why today's tumult in the global markets should have shocked anyone.  Okay, the markets bet against the Brexit and, as has happened numerous times in the past, the markets were wrong.  All the Wall Street geniuses, as well as the now defunct British PM, miscalculated the sentiments of the British people.  So the Dow fell 610 points.  It was grossly oversold anyway.  And, after all, though the news came crashing in today, Britain's actually exit doesn't happen for two whole years; and the whole universe can change in two years.

It's a reasonable forecast that there will be extensive and constructive negotiations between England and Europe and that the markets will adapt.  This Brexit does not have to portend a global recession.  Smart investors instead should look at today's events as an almost perfect storm of buying opportunities and put some of this excess cash on the sidelines to work though, to be sure, it is likely to be a while before the full impact of last night's vote can be assessed.  Volume was huge, over 15 billion shares traded, nearly triple the average. But much of this was due to index reconstitution.  One thing I've never understood is why, especially on really important days like today, they cannot net out these double-dipping obstructions and tell us what the real trading was.  Oh well.  The markets have a whole weekend now to figure out the next move so we'll have a much more accurate picture come Monday.

Markets | Fri Jun 24, 2016 8:30pm EDT

Worst day in 10 months as Wall Street reacts to 'Brexit'

NEW YORK | BY RODRIGO CAMPOS

DJ:  17,400.75  -610.32       NAS: 4,707.98  -202.06        S&P: 2,037.41  -75.91

The S&P 500 turned negative for the year-to-date on Friday as Wall Street suffered its largest selloff in 10 months after Britain's decision to leave the European Union caught traders wrong-footed.

The British pound touched a 30-year low versus the U.S. dollar and stocks dropped sharply across the globe on fears the decision could hurt investment and usher in many months of both political and economic uncertainty.

"Expect weaker investment and thereby slower economic growth to persist during the 2-3 year negotiations to leave the European Union," Deutsche Bank economists said in a Friday note.

Equity futures neared an 11-month high to start the overnight session as markets wrongly bet that the "Remain" camp would prevail in Britain's referendum, but sold off sharply as the results showed otherwise - even triggering a market stop put in place to curtail volatility.

The decline during regular market hours seemed more orderly, but the S&P 500 financial sector .SPSY posted its largest percentage decline since November 2011, tumbling 5.4 percent. The S&P 500 lost all the year's gains and suffered its largest decline since late August last year.

The S&P managed to close in the area of what analysts called significant technical support near 2,040, but many expect the next weeks to remain volatile.

The CBOE Volatility index .VIX ended up 49 percent at 25.76, its highest level since Feb. 11 - when equities hit their lows of the year.

"The market has really not fully digested the second-order impacts of this," said Stephen Auth, chief investment officer at Federated Investors in New York.

"We are keeping our clients in dividend stocks and on a defensive strategy," he said. "That's the story until we reach better risk-reward levels," which Auth said could be near the S&P 500's February low of 1,830.

High-dividend-paying utilities .SPLRCU were the only S&P 500 level to end the day in the black, with a meager 0.09 percent gain.

The Dow Jones industrial average .DJI fell 611.21 points, or 3.39 percent, to 17,399.86, the S&P 500 .SPX lost 76.02 points, or 3.6 percent, to 2,037.3, and the Nasdaq Composite .IXICdropped 202.06 points, or 4.12 percent, to 4,707.98.

Many market participants, however, saw the broad-based decline as an opportunity for investors on the sidelines to gradually increase their stock holdings.

"What I see going to happen is the Federal Reserve will not raise rates in 2016, that is off the table and a market positive," said Doug Cote, chief market strategist at Voya Investment Management in New York.

"If you have cash on the sidelines it could be a buying opportunity but I wouldn't be changing (bonds-stocks) allocations right now," he said, adding he expects other major central banks to also give the market support.

The weekly declines in the S&P 500 and Dow industrials, both off 1.6 percent, were the largest since February, and the Nasdaq's 1.9 percent drop was marginally lower than the previous week's.

The relatively less severe weekly decline points to an unraveling of trades put in place after polls last weekend showed increasing momentum for the "Remain" camp in the British referendum.

The pound GBP= had breached $1.50 versus the U.S. dollar but later tumbled to near $1.32, its lowest level since 1985. Traders were closely following moves in currencies, calling them the contagion mechanism between markets.

Volume in U.S. exchanges hit 15.2 billion as the trade-heavy reconstitution of the Russell indexes crossed at the market close. It was the highest volume since August 2011 and compares with the daily average of 6.8 billion over the last 20 trading sessions.

Declining issues outnumbered advancing ones on the NYSE by a 5.47-to-1 ratio, and on the Nasdaq, a 5.20-to-1 ratio favored decliners.

The S&P 500 posted 28 new 52-week highs and 12 new lows; the Nasdaq recorded 33 new highs and 107 new lows.