* S&P 500 on track for biggest weekly decline of 2013
* Energy shares likely to drop, oil slumps 1 percent
* F5 Networks plunges after weak outlook, weighs on peers
* Indexes down: Dow 1 pct, S&P 1.1 pct, Nasdaq 1.3 pct
By Angela Moon
NEW YORK, April 5 (Reuters) - U.S. stocks fell on Friday, with major indexes down more than 1 percent, in the wake of a payrolls report that was much weaker than expected, the latest in a series of data to indicate economic growth may be losing momentum.
The S&P 500, currently down about 1.7 percent for the week, was on track for its worst weekly decline of the year. Losses were broad, with more than 80 percent of stocks on the New York Stock Exchange and Nasdaq trading in negative territory.
About 88,000 jobs were added in March, less than half the forecast of 200,000, though the unemployment rate dipped to 7.6 percent from 7.7 percent. The report follows similarly disappointing reads on the manufacturing and services sector, as well as other poor labor market data.
"It would be as much of a mistake to overreact to this number to the downside as it was to overreact to the February data to the upside," said Steve Blitz, chief economist at ITG Investment Research.
"What we have in today's employment report is what the ISM data and the auto (not truck) sales indicated - the economy is expanding but not accelerating."
The Dow Jones industrial average was down 144.62 points, or 0.99 percent, at 14,461.49. The Standard & Poor's 500 Index was down 17.11 points, or 1.10 percent, at 1,542.87. The Nasdaq Composite Index was down 43.25 points, or 1.34 percent, at 3,181.73.
F5 Networks Inc was the S&P's biggest percentage loser, dropping nearly 20 percent to $72.55, a day after forecasting second-quarter earnings and revenue that were well below expectations.
Several of F5's peers were also sharply lower, with Juniper Networks off 4.6 percent at $17.27 and Citrix Systems down 2.6 percent at $67.90.
Retailers weakened, with the SPDR S&P Retail exchange-traded fund down 1.1 percent at $69.42.
If the S&P closes down on the week, it will be only the third weekly loss this year for the index, which has struggled to surpass an intraday record high of 1,576.09. The index has gained about 8 percent this year without a significant pullback, leading many analysts to call for one.
Equity market gains have been partially fueled by a bond-buying program by the Federal Reserve. Measures from central banks around the world have also helped, and on Thursday, Wall Street rose after the Bank of Japan announced aggressive policies to jump-start its economy.
Friday's payroll report "should reinforce the Fed's recent bond buying activity; but that may not be enough to turn today's bearish feelings in the markets," said Todd Schoenberger, managing partner at LandColt Capital in New York.
Energy shares were pressured, as the group is closely tied to economic growth expectations. Crude oil fell for a third straight day, dropping 0.6 percent and extending its decline for the week to more than 5 percent. Chevron Corp fell 1 percent to $116.85.
Overseas, European shares slumped 1.9 percent, falling to a one-month low on concerns over growth.
Earnings forecasts have been scaled back heading into first-quarter reports, due to be kicked off next week by Alcoa . S&P 500 earnings are expected to have risen just 1.6 percent from a year ago, according to Thomson Reuters data, down from 4.3 percent forecast in January. Alcoa stock was down 0.1 percent at $8.21.
Geopolitical tensions will remain in focus after North Korea placed two of its intermediate range missiles on mobile launchers and hid them on the east coast of the country, South Korean media reported on Friday.