The U.K. and the European Union must work together and quickly to mitigate the impact on markets of Britain's Brexit vote, the head of the International Monetary Fund said.

The IMF's Christine Lagarde said on Sunday night that the fund "will continue to encourage the parties involved to actually proceed with this transition in the most efficient, predictable way in order to reduce the level of uncertainty, which will itself determine the level of risk we are facing."

Speaking at the Aspen Ideas Festival in Colorado, Lagarde said, "At this point in time, policy makers, both in the U.K. and in Europe, are holding that level of uncertainty in their hands. And how they come out in the next few days is going to really drive the direction in which risks will go."

She added that the turmoil in the Labour and Conservative parties is adding uncertainty to the market. Lagarde also said that markets mispriced a Brexit. "Despite that, there was no panic. There was a violent, brutal, immediate massive move...But there was no panic, and the central bankers did the job that they were prepared to do just in case," she said.

This comes as S&P said that $2.08 trillion was wiped off global markets on Friday. It said this was the biggest daily loss ever - bigger than that triggered by the Lehman Brothers bankruptcy during the financial crisis and the Black Monday crash in 1987.

Calculations based on S&P Global Broad Market Index - based on equity markets in 47 countries - showed that 4.7% was taken off equity indices' market capitalizations on Friday. Benchmark indices in Europe were the hardest hit, with those in Italy and Spain falling about 12%.

Markets today have seemingly calmed. The FTSE 100 opened only slightly down and was recently at 6,113.12, 0.41% down.

The pound was recently down 1.58% against the dollar at $1.3463.