Equities, oil, and the euro moved lower overnight, as euro-zone leaders prepared to restart negotiations on a new bailout package for Greece after the country's voters' gave a resounding "no" in Sunday's referendum to the most recent proposal.

Euclid Tsakalotos took over as Greek finance minister following the resignation of Yanis Varoufakis after a fractious few months leading talks with his European counterparts.

"Soon after the announcement of the referendum results, I was made aware of a certain preference by some Eurogroup participants, and assorted 'partners', for my... 'absence' from its meetings; an idea that the Prime Minister [Alexis Tsipras] judged to be potentially helpful to him in reaching an agreement. For this reason I am leaving the Ministry of Finance today," Varoufakis said in a statement.

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Greece was not solely to blame for a slide in oil as West Texas Intermediate crude oil sank more than 7 percent. Over the weekend, China announced emergency measures to stem its stock market slide, while Iran might soon add to the world's oversupply of oil.

"Even without Greece, China's stock market woes and Iran priming to hit the market with more barrels, the demand picture in oil has only been okay while the supply picture has been phenomenal," John Kilduff, partner at New York energy hedge fund Again Capital, told Reuters.

"With these number of bearish elements weighing on the market now, the only thing of support has been the seasonal demand in gasoline, and even that will be going away soon," Kilduff noted. This past weekend marks the traditional start of the US summer holiday season.

Wall Street moved lower as had markets in Europe. In late afternoon trading in New York, the Dow Jones Industrial Average fell 0.64 percent, the Standard & Poor's 500 Index dropped 0.67 per cent, and the Nasdaq Composite Index retreated 0.73 per cent. Last Friday US markets were closed for the Independence Day holiday.

Video:Economist: Greek minister resignation 'extremely strong signal'

Declines in shares of General Electric and those of Intel, each trading 1.9 percent lower, led the Dow lower.

Europe's Stoxx 600 Index finished the session with a 1.2 percent decline from the previous close. The UK's FTSE 100 Index fell 0.8 percent, Germany's DAX gave up 1.45 percent, while France's CAC 40 Index sank 2 percent.

"The referendum doesn't really change anything with the negotiations," Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia, told Reuters. "I think after the initial knee jerk reaction, the majority opinion is that there is still a possibility of some sort of a deal that keeps Greece in the euro zone."

Bill Gross, manager of the Janus Global Unconstrained Bond Fund, warned that Greece is in "the eye of the hurricane" and that he sees a 70 percent to 80 percent probability of a Greek exit from the euro.

"I do not believe the situation really is calm," Gross told Bloomberg in an interview, adding he was surprised by the muted market reaction to the outcome of the Greek referendum.

The M&A train on Wall Street keeps moving. Aetna agreed to buy rival Humana for about US$35 billion in cash and stock. Shares of Aetna last traded 7.7 percent lower, while those of Humana traded 1 percent higher.

There was good news on the US economic front. The Institute for Supply Management's non-manufacturing index rose to 56 in June, from 55.7 in May.