U.S. futures fell slightly Sunday evening as investors braced for more fallout from the Brexit vote in the U.K.

The S&P 500 futures index fell about 0.47%, to 2009.00, at 7 p.m., after dropping 3.6%, to 2037.41 in regular trading Friday. Dow Jones Industrial Average and Nasdaq futures also fell, down 0.46% and 0.62%, respectively. Both indices also fell sharply Friday.

European currencies also suffered in trading Monday morning in Tokyo. Sterling continued to drop, down 1.6%, to $1.3468, while the yen rose slightly to 102.19 per dollar. As investors fled to safe assets, the yen appreciated against both the dollar and the pound, moving below 100 yen per dollar for the first time since November 2013.

In Tokyo, the Nikkei rose 1.15% Monday morning, to 15,123.51 at 9:29 local time. Japanese stocks fell in the immediate aftermath of the Brexit vote as the yen strengthened; as an export-driven economy, Japan is hurt by the yen's strength.

Goldman Sachs (GS) - Get Report analysts wrote in a note that Brexit is likely to trigger a recession in the U.K., and the potential political fallout could be devastating across Europe.

"Even if -- or perhaps especially if -- the U.K. economy holds up better than we expect, the political risks to the integrity of [the] remainder of the European Union have grown significantly," analysts wrote. "Nationalist parties in other European countries -- especially France, the Netherlands, and the Czech Republic -- are already clamoring for referenda of their own." Further exits from the EU "could raise renewed questions about the future of the euro area that might necessitate much more aggressive ECB intervention."

Brexit's impact on the United States market, however, will be modest, the report added: "The Brexit shock is a regional event, not a global one."

Conservative Member of Parliament Boris Johnson, one of the leading voices of the pro-Leave  movement, sought to calm investors in a column in The Telegraph newspaper after Brexit fears sent markets reeling.

"The pound remains higher than it was in 2013 and 2014," he wrote, inaccurately. The lowest the pound went at that time was $1.48. The pound slid to a 31-year low against the U.S. dollar after the U.K. voted Leave.

Attempting to allay concerns about isolation from the European Union, he wrote that the U.K. "is part of Europe, and always will be." Once the country leaves the EU, he asserted, its relationship with Europe will largely remain unaltered.

"British people will still be able to go and work in the EU; to live; to travel; to study; to buy homes and to settle down," Johnson continued. "There will continue to be free trade, and access to the single market. Britain is and always will be a great European power, offering top-table opinions and giving leadership on everything from foreign policy to defence to counter-terrorism and intelligence-sharing -- all the things we need to do together to make our world safer."

MPs from the majority Conservative Party, including Johnson, are jockeying to replace departing pro-Remain Prime Minister David Cameron, who said he will step down.

Chancellor of the Exchequer George Osborne, a close ally of Cameron's, is expected to deliver a statement Monday morning before the markets open.

The head of the rival Labour Party, Jeremy Corbyn, grappled with an exodus of high-level members of the party over the weekend. Corbyn abruptly dismissed Shadow Foreign Secretary Hilary Benn late Saturday evening amid reports that Benn was planning a coup against Corbyn. On Sunday, at least 11 of the 30 remaining members of the shadow cabinet also resigned.

The political upheaval in the U.K. was not unique this weekend. Spain has not had a government since December, and a second general election proved inconclusive. Incumbent Prime Minister Mariano Rajoy of the pro-austerity People's Party won a plurality. Although he lacks a majority, Rajoy is better positioned to form a coalition government with a rival party.

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See full coverage of the Brexit debate here.