A protester, and supporter of Senator Bernie Sanders march in opposition to Hillary Clinton ahead of the Democratic National Convention in Philadelphia, Pennsylvania, US, July 24, 2016. [Photo/Agencies]

The emails angered many Sanders supporters who were already dismayed by Clinton's choice on Friday of low-key US Senator Tim Kaine of Virginia as her vice presidential running mate. Kaine, 58, who could appeal to independents and moderates, has never been aligned with party liberals.

Sanders, who has endorsed Clinton and will speak on her behalf to the convention on Monday, said he would have preferred she pick US Senator Elizabeth Warren of Massachusetts, a favorite of the party's liberal wing, as her No 2.

"I have known Tim Kaine for a number of years. ... Tim is a very, very smart guy. He is a very nice guy," Sanders said on NBC's "Meet the Press."

"He is more conservative than I am. Would I have preferred to see somebody like an Elizabeth Warren selected by Secretary Clinton? Yes, I would have," he said.

Carrying pitchforks meant to portray Clinton as the devil, hundreds of Sanders supporters took to the streets of Philadelphia earlier on Sunday to say they felt betrayed by the DNC.

"It just validated everything we thought, everything we believed to be true, that this was completely rigged right from the beginning, and that you know it was really about what they were doing everything to set it up so she would win," Sanders supporter Gwen Sperling said.

DNC Vice Chairwoman Donna Brazile will serve as interim chair through the election, the DNC said on Twitter.