Three days prior to Greta Thunberg’s surprise visit to Iowa City on Friday, the organizer and local climate activist, Massimo Biggers, a 14-year-old Iowa City high school student, was preparing to strike – as he has done every Friday, sometimes on his own, since the Global Climate Strike day Thunberg inspired on 15 March.

Out of the blue, a message arrived from the Swedish teen activist, with whom he had been in touch, asking him if he was planning to strike again this Friday. “Of course!” he replied, and for the last 48 hours, according to his father, Jeff, neither had slept. “This was truly a miracle to have the town pull this together,” he said.

More than 3,000 people gathered at short notice in the shadow of the University of Iowa on Friday afternoon to hear Thunberg speak.

Attendees participate in a climate strike rally with climate change environmental teen activist Greta Thunberg, in Iowa City, Iowa. Photograph: Daniel Acker/Reuters

“Wow” said Thunberg, squinting in the sun, as the crowd chanted her name. “It’s just so many people I don’t think any of us expected this many people. This is real hope, so many people gathering on a weekday at such short notice. This is real hope to me,” she said.

Biggers has spent the last six months mobilizing fellow students to pressure the city council into adopting more stringent measures to address climate change.

“At the time our specific goal was to get the school board to pass a climate resolution,” he said in an interview with the Guardian. “But then it was pretty easy to get the school board to get a climate resolution so we went to the city council and now we’re trying to get the coal fired [power] plant shut down,” he added.

The University of Iowa burns coal at its power plant, providing a clear target for the young activists who lead the crowd in regular chants of “End Coal Now!”

“To be part of a global movement and especially at this stage, it’s just an amazing, amazing opportunity to see a global leader,” said Abbey Jordahl, a freshman at UI from Ankeny, Iowa. “Honestly I couldn’t believe it when I read that Greta was coming here, I was like … Iowa City?! What?! I’ve been following her for a while, honestly I can’t believe I’m here!”

Activist Greta Thunberg joins a climate strike march in Iowa City. Photograph: Daniel Acker/Reuters

Thunberg rallied an enthusiastic, young crowd, flush with high school and university students, many of whom had made their own signs to greet her. “Right now the world leaders keep acting like children and somebody needs to be the adult in the room,” she said, referencing her speech at the United Nations in New York the previous week.

Thunberg said afterwards that she doesn’t like being the centre of attention. “It’s very hard to get used to,” she admitted, “but it’s something I have to do and I can’t really complain. I just kind of shut off the brain,” she said.

Explaining why she believes she is now on the receiving end of criticism from world leaders like Donald Trump and Vladimir Putin, she suggested: “They do it because they see something that is threatening and they want to silence that and if they don’t have anything else to say, if they can’t criticize the science, which is all we are saying, then they start attacking us personally and sending threats and hate and so on so maybe they can’t cope with it.

“It could definitely be they feel threatened by a new generation,” she added “There is a lot of young people especially young girls in this movement who are leading and maybe they don’t like that, I don’t know!”

Claire Carlson, a senior studying geoscience, environmental science and business at UI from Ottumwa had joined friends from the Environmental Science Club at the rally. “Its nice to finally see a large group of people come together and address the problem,” she said. “I think Greta’s done a really good job of bringing a voice to that especially at a really young age. I think it means that more people are starting to understand the need for impactful legislation with regards to climate change.”

More than 3,000 people gathered in the shadow of the University of Iowa on Friday afternoon to hear Thunberg speak. Photograph: Daniel Acker/Reuters

Carlson, like her friend Emma Hartke, will be able to caucus next February for a Democratic candidate. “I think Elizabeth Warren has a really good plan for climate change,” said Hartke. Carlson favours the young mayor from South Bend, Indiana, Pete Buttigieg, but Warren is the name on most people’s lips when asked which way they’re currently leaning.

“There are still some centrists who like to play both sides on climate … such as Joe Biden,” said Derek Torres who has moved to Iowa City from Atlanta. “They always speak in terms of jobs … but if there’s no air or water there’s no jobs,” he reasoned.

Thunberg has become adept at using Twitter to spread her message, but does she enjoy using it? “Yes and no,” she said. “A lot of it is just meaningless. And people wanting to brag about themselves or whatever, lots of hate, trolls, anonymous people who have separate accounts.” But she admitted she does enjoy updating her tagline. “You need to have fun as well,” she joked.