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It's going to get ugly, says Dennis Gartman

Investor should brace themselves because things could get ugly, widely followed investor Dennis Gartman said Thursday.

He thinks a 5 percent to 8 percent market correction is in the cards.

"It's not going to get ugly bad; it's not going to get ugly for a long period of time. I think it's going to get ugly swiftly and I think it's going to make a lot of people very nervous," the editor and publisher of The Gartman Letter said in an interview with CNBC's "Closing Bell" on Thursday.

He expects the downturn to last two to three weeks, and said then it'll be time to buy again.

"It's still a bull market and as I've always said, in a bull market there's only three things you can do: be really long, be pleasantly long or be neutral. I think it's time to be neutral."

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U.S. stocks closed down more than 1 percent Thursday, with the Dow Jones industrial average dropping more than 250 points before recovering some of those losses. The dip temporarily wiped out gains for the year. The Nasdaq briefly fell more than 2 percent and held below its 50-day moving average in the close.

The higher price of oil and weakness in the U.S. dollar and were part of the reason stocks sold off, Gartman said.

He noted the dollar was slammed against the euro, but was strong relative to the Japanese yen, Canada dollar and British pound sterling.

"Too many people were terribly short of the euro against everything.… They're unwinding that," he noted.

"That may last for a couple of days. That could really get ugly."

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—CNBC's Evelyn Cheng contributed to this report.