A Donald Trump win could spark an immediate sell-off of up to 5 percent for the S&P 500, according to analysts at Citi, who also warn on slower growth or even recession for the U.S.

According to the latest note out from global equity strategists at Citi Friday, fears for stocks after a Trump victory are two-fold.

"A Trump win risks slower growth or recession if trade is restricted and fiscal expansion plans curtailed. Uncertainty alone could hit the economy. Global growth will also be impacted if uncertainty rises, U.S. growth is hit and U.S. financial conditions tighten," Citi warned.

"If Donald Trump were to win, that outcome would have been unexpected and thereby may cause a jump in the equity risk premium with negative P/E (price –to-earnings) multiple repercussions. We think a Trump victory could spark a 3-5 percent setback in the S&P 500," summarizes the Citi note.

The price-to-earnings ratio is a key valuation metric for stocks which gauges how much an investor must pay on the stock market for each cent of earnings, CNBC.com reported.

Citi is far from alone in warning about the risks to the markets.

Executives of Paul Singer's hedge fund Elliott Management, in a letter to investors seen by CNBC's Kate Kelly, warn that a rapid inflation is the $30 billion hedge fund's biggest concern in the current environment.

They warned that such a spike would not only collapse bond prices, but potentially lead to a stock market crash.

"This may seem like a strange thing to worry about under the current circumstances, but the tide toward inflation could turn rather abruptly," wrote the money managers in their third-quarter letter, dated Oct. 28. "If inflation starts accelerating to an annual rate of high single digits or greater, it will be quite difficult for the mix of strategies that Elliott favors to 'keep up,'" they said.

"Every sniffle is being treated by central banks as acute respiratory distress syndrome worthy of 'code-blues' and teams of frantic pumpers and fixers," the managers wrote.

"What this policy landscape has engendered is a widespread belief, or at least a strong suspicion, that stock and bond prices won't ever be allowed to go down in any meaningful way."

This mentality, the writers added, "has encouraged massively risky behavior."

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