PHILADELPHIA — Diehard Bernie Sanders supporters hijacked the opening day of the Democratic convention on Monday — drowning out speakers with chants of “Bernie! Bernie!” while loudly booing the mention of Hillary Clinton’s name.

The first outburst erupted even before the opening prayer was finished, when the Rev. Cynthia Hale mentioned the former secretary of state in her invocation — and the Sanders’ delegates booed lustily.

When Ohio Rep. Marcia Fudge, the convention chair, took the stage, she was quickly drowned out by protesters opposed to the Trans-Pacific Partnership and still angry over the e-mail scandal in which party big shots plotted to sabotage Sanders’ candidacy.

“We are all Democrats and we need to act like it,” Fudge lectured the unruly crowd — to no avail.

Maryland Rep. Elijah Cummings spoke minutes later, and the crowd began shouting, “No TPP! No TPP!” — voicing their opposition to the trade deal that Clinton supported but later backed away from.

The protests prompted Sanders to send an e-mail to his delegates, pleading for unity and warning that the tumult would only play into Donald Trump’s hands.

“I would ask you as a personal courtesy to me to not engage in any kind of protest or demonstration on the convention floor,” he wrote.

In another move meant to douse the political fire, the DNC released a statement apologizing for comments by party bigs disparaging Sanders in the leaked e-mails, which led party chair Debbie Wasserman Schultz to resign Sunday.

But many Sanders backers were adamant.

“I wouldn’t vote for [Hillary] for dog catcher. No, never in a million years,” said Michigan delegate Melissa Arab.

After the rocky start, the party rolled out three of its biggest guns — First Lady Michelle Obama, Massachusetts Sen. Elizabeth Warren and Sanders himself — to tamp down the revolt.

“It is no secret that Hillary Clinton and I disagree on a number of issues,” Sanders told the crowd, which gave him the longest ovation of the night.

“Our job now is to see that platform implemented by a Democratic Senate, a Democratic House and a Hillary Clinton presidency — and I am going to do everything I can to make that happen.”

Former President Bill Clinton leapt to his feet and applauded, as did most of the delegates.

But not everyone. Scattered boos filled the arena.

Warren, the leftist darling, drew cheers when she slammed Trump as a greedy con man who is out only for himself.

But she also came in for some rebukes for boosting Hillary, with shouts of “We trusted you!”

The uproar was the last thing Democrats wanted on the first day of the four-day convention, which leaders had hoped would show a unified party focused on defeating Trump.

Comedian Sarah Silverman — a Sanders supporter — said she was proud to “be a part of Bernie’s movement” but said now that it was essential to elect Clinton.

“Can I just say to the Bernie-or- bust people, you’re being ridiculous,” she scolded, prompting his backers to chant, “Bernie! Bernie!” for nearly a minute.

“Listen to what you did” comedian-turned-Minnesota Sen. Al Franken reproached her.

Before the convention began, furious supporters actually booed Sanders as he urged them at a morning meeting to switch teams to help Clinton overcome Trump.

Wasserman Schultz was also booed when she addressed her home-state delegation of Florida, a day after announcing she would resign as DNC chair.

The reception that greeted Wasserman Schultz — who tried to ignore it with repeated “thank yous” — was so angry that she was forced to change her mind and not gavel in the convention.

Outside the Wells Fargo Center, hundreds of Sanders supporters marched in the sweltering heat, angrily chanting, “Nominate Sanders or lose in November.”

Cops said 55 people were taken into custody.

Additional reporting by Marisa Schultz and Bob Fredericks