Yields hit fresh lows amid violent Greek protests

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Yields extended earlier losses on Wednesday as violence erupted in Athens ahead of a parliamentary vote on whether Greece will accept creditor-proposed reforms in exchange for a third bailout.

The vote is expected to pass, but the atmosphere's tense, with multiple politicians from within Prime Minister Alexis Tsipras's own party opposing the deal.

Benchmark 10-year note yields were last down 5 basis points to session low of 2.348 percent, from 2.40 percent late on Monday.

Yields slipped earlier after on Wednesday after the Federal Reserve said economic activity expanded at a modest pace between mid-May and June.

In its Beige Book report of anecdotal information on business activity collected from contacts nationwide, the U.S. central bank said most of its regional Fed banks described growth as either progressing at a "moderate" or "modest" pace.

It said manufacturing activity was uneven across the country, with lower oil prices hurting the oil and gas industry, and a strong dollar dampening exports.

Separately, Federal Reserve Chair Janet Yellen said the U.S. central bank remains on track to raise interest rates this year, keeping expectations that the Fed may make its first hike in September.

"If the economy evolves as we expect, economic conditions likely would make it appropriate at some point this year to raise the federal funds rate," Yellen said in testimony prepared for the U.S. House of Representatives Financial Services Committee, affirming the view of a central bank prepared to gradually raise rates after more than six years at a near-zero level.

Read More Live: Yellen under attack on Capitol Hill

The comments mirror a speech by Yellen on the economy on Friday, which also sent yields higher.

"Today's remarks are similar in tone," said Tom Porcelli, chief U.S. economist at RBC Capital Markets in New York. "I think it's interesting that she notes the economy could snap back faster, as headwinds start to fade, and then she goes on to basically say as a result its appropriate for us to start liftoff."

Positive economic data on Wednesday also supported the view that the Fed will raise rates later this year.

Read MoreGundlach at Delivering Alpha: Fed's not hiking

The Labor Department said on Wednesday its producer price index for final demand increased 0.4 percent last month after increasing 0.5 percent in May. It was the second straight month of increase in producer prices.

Other data on Wednesday showed a rebound in factory activity in New York state this month. U.S. factory production, however, failed to advance for a second straight month in June, the Federal Reserve also said on Wednesday.

—CNBC contributed to this report.