The first day of the Democratic National Convention kicked off with a massive gathering of Bernie Sanders supporters taking to the streets in Philadelphia in protest of Hillary Clinton’s nomination as the party’s pick for the White House.

This isn’t the first time that the Bernie or Bust movement has voiced its displeasure with Clinton, but today’s protests came after Sanders had already officially endorsed Clinton and urged his supporters to do the Democratic thing and support her. While there’s nothing that Sanders can do to prevent his fans from demonstrating, the senator sent out an email today urging the delegates gathered in Philly to pull it together and refrain from booing Clinton during tonight’s speeches.

“During this convention you represent not just the 13.3 million who voted for our campaign. You also represent the future of our movement,” Sanders wrote. “I am asking all of us to bear that in mind while we are on the floor of the convention. Our credibility as a movement will be damaged by booing, turning of backs, walking out or other similar displays.”

We have got to defeat Donald Trump and do everything we can to elect Hillary Clinton to the White house. #DemsInPhilly — Bernie Sanders (@BernieSanders) July 25, 2016

Only time will tell if the Sanders supporters will get in formation behind him.

Read the full text of the letter: