US stocks tumbled Monday, joining global equity markets in retreat on the looming threat of a Greek debt default.

The Dow Jones Industrial Average fell 350.33 points (1.95 percent) to 17,596.35.

The broad-based S&P 500 sank 43.85 (2.09 percent) to 2,057.64, while the tech-rich Nasdaq Composite Index dropped 122.04 (2.40 percent) to 4,958.47.

Greece shuttered its banks and imposed capital controls to halt panic cash withdrawals ahead of a July 5 referendum on creditor proposals.

Standard & Poor's downgraded Greece's credit rating deeper into junk territory and estimated the odds at 50 percent of the debt-wracked country leaving the eurozone.

"There is a lot of nervousness in terms of how all this is going to play out," said Bill Lynch, director of investment at Hinsdale Associates.

But Alan Skrainka, chief investment officer at Cornerstone Wealth Management, said Monday's losses were far from catastrophic.

"I believe markets outside of Greece have responded in a relatively calm way given the surprise nature" of Greece's announcements, he said.

The Greek economy is relatively small and a default "offers very little risk for the global economy or for the rest of Europe."