"I am back," Democratic presidential hopeful Bernie Sanders said Saturday, as he held a packed rally in New York City that featured Rep. Alexandria Ocasio-Cortez, who endorsed the Vermont senator's White House bid.

Watch video of the event below: Starts around 18:20

The "Bernie's Back" event, which kicked off shortly after 1 pm in Queensbridge Park in Queens, marked the campaign's first rally since Sanders was hospitalized for a heart attack earlier this month. Sanders told the crowd he was "more ready than ever to carry on with you the epic struggle that we face today" and "more than ready to assume the office of president of the United States."

On Saturday afternoon, the hashtag #BerniesBack was trending in the U.S.

Ocasio-Cortez's endorsement was reported earlier this week by the Washington Post.

The capacity for the rally was 20,000, but so many people came the campaign had to turn people away, said Sanders. According to the campaign, nearly 26,000 people were in attendance.

"Our priority is not only defeating Donald Trump," Ocasio-Cortez told the crowd. "It's defeating the system of which he is a symptom."

The two lawmakers sung each other's praises. AOC praised Sanders for his "non-stop advocacy" including fighting for a single-payer healthcare system while she was a child relying on CHIP.

Sanders, meanwhile, called Ocasio-Cortez a "fierce defender of the working class of our country" who's "taken on the greed of Wall Street" and "electrified this country" with her Green New Deal legislation.

At the rally, Sanders said in a tweet Friday night, "we will say, without apology: Safe and decent housing is an essential right. A Green New Deal is the only sufficient response to our climate crisis. And the obscene greed of corporate America must end."

On social media, the campaign shared remarks from other speakers at the rally, including campaign co-chair and San Juan, Puerto Rico Mayor Caren Yulín Cruz, filmmaker Michael Moore, and campaign co-chair Nina Turner.

Speaking to NBC News Saturday before the rally began, Ocasio-Cortez, who was an organizer for Sanders' 2016 campaign, said her endorsement was due to "a moment of clarity" about the role she wanted to play. She said she decided, "I want to be part of a mass movement."

"It was less about... being overly politically calculating," she said, "and it was more about the authenticity and clarity of that moment."

Sanders has also received the endorsement of two other members of the Squad—Reps. Ilhan Omar (D-Minn.) and Rashida Tlaib (D-Mich.)

Explaining her support, Omar said in a video released this week: "The senator is the only candidate that is proposing a complete cancellation of student debt. The senator is the only candidate that is proposing to provide universal school meals. The senator is the only candidate that wants to make sure that we end our endless wars and will fight for human rights and hold everyone accountable regardless of whether they are an ally or a foe."

In a phone call with supporters on Friday, Sanders said the rally would be a "powerful event" and touched on key issues of his campaign including income and wealth inequality, Medicare for All, and the climate crisis.

The campaign, added Sanders, has "a great path to victory."

That path relies on generating "the largest voter turnout by far in the history of our country" to counter President Donald Trump, whom Sanders called "the most dangerous president in the history of this country," as well as a "formidable opponent" who will have "unlimited amounts of money from his billionaire friends and from others."

That makes the endorsements from Ocasio-Cortez, Omar, and other progressives so key, said Sanders, as "they are able to generate and are generating enormous support among young people, among people of color, among working class people all over this country."

"With Alexandria, with Ilhan, and others on board," Sanders continued, "we're going to put together a coalition that is going to win and win big" and be able to create "an economy and a government that works for everybody and not just the one percent and wealthy campaign contributors."