US stocks finished modestly lower Wednesday, snapping a four-day winning streak after violent protests outside the Greek parliament revived anxiety about the eurozone.

At the closing bell, the Dow Jones Industrial Average was essentially flat at 18,053.44, down 0.14 of a point.

The broad-based S&P 500 lost 1.24 (0.06 percent) at 2,107.71, while the tech-rich Nasdaq Composite Index fell 5.95 (0.12 percent) to 5,098.94.

US stocks were in positive territory most of the day, but an afternoon sell-off was triggered by images of riot police spewing tear gas as hooded protesters hurled petrol bombs at them outside the Greek parliament, while lawmakers inside debated whether to accept a European bailout package to stave off a debt default.

"What was priced into the market was a smooth agreement" in Greece, said Art Hogan, chief market strategist at Wunderlich Securities.

"That violent protest outside the parliament and the timing of the rollover in the market (into negative territory) is identical," he said.