By Josie Cox and Andrea Tryphonides 

Jitters over the financial future of Greece roiled global markets on Friday, with equities falling sharply and demand surging for assets considered safest during times of stress such as German government bonds.

The Stoxx Europe 600 index ended the day 1.7% lower, mirroring similar losses across individual country indexes. Athens' main stock exchange fell 3%, taking declines so far this year to almost 12% and over the last 12 months to over 41%, making it one of the world's worst performing indexes.

In the U.S., stocks opened lower and continued selling off in early trading. The Dow Jones Industrial Average recently was off 1.5%, or 263 points, at 17,842. The declines in the Standard & Poor's 500 and Nasdaq Composite were more moderate.

Strategists said that a lack of progress in negotiations between beleaguered Greece and its international creditors had substantially increased the risk of Greece defaulting on its debt and even exiting the euro.

"In the absence of a deal in the next few weeks, the government might not be able to avoid default, which, we fear, would likely raise the risk of Grexit," economists at UBS wrote in a note.

"The government's budget situation is increasingly precarious while the negotiations on the reform program are continuing only very slowly," they added.

"Athens will have to give in to its creditors demands" if it wants to avoid default, said Eirini Tsekeridou, a fixed income research analyst at Swiss private bank Julius Baer, adding that only "real gamblers" would currently have any exposure to the country.

On Friday, the yield on Greece's 10-year bonds was at 12.71%, close to a two-year high, while two-year yields were at 26.37%, close to their highest since being issued. Yields fall as bond prices rise.

An inverted yield curve, where shorter-term debt yields more than longer-dated bonds, signals that investors foresee a very high risk of default.

Exacerbating Friday's selloff in equities and ballooning risk-aversion were fears surrounding China, with the world's second largest economy allowing fund managers to lend stocks for short-selling to increase the supply of shares, the Securities Association of China said on its website on Friday.

The Chinese stock market slumped over 5% in post-close trading, weighing on sentiment in Europe.

"Monday seems like a long way away at the moment, but there are fears that we could see a sharp sell off in Asia at the start of the week, which markets in Europe already appear to be anticipating," said Jeremy Batstone-Carr, chief economist at Charles Stanley in London.

German government bonds, broadly seen as a low-risk haven for investors during times of stress, rose sharply to fresh record highs, extending gains earlier this week which were triggered by the European Central Bank reiterating its commitment to a massive quantitative easing program.

The yield on the country's benchmark 10-year government bond traded at 0.05%, breaking through Thursday's all-time low. Investors said that it was now only a matter of time before the yield on those bonds turns negative.

In currency markets, the euro was trading marginally lower against the dollar at $1.075, though traders said this was more to do with dollar strength than euro weakness, after data showed that U.S. consumer prices increased for the second consecutive month in March. By contrast, figures showed that consumer prices across the European Union fell for a fourth straight month in March.

In commodity markets, Brent crude lost 0.1% to trade at $63.88 a barrel. Gold, also considered a safe asset in unstable market conditions was up 0.5% at $1,203.90 per troy ounce.

Write to Josie Cox at josie.cox@wsj.com and Andrea Tryphonides at andrea.tryphonides@wsj.com