SYDNEY, Australia — Australia’s most prominent female Muslim activist, an outspoken critic of her country’s immigration policies, was denied entry into the United States on Wednesday.

Yassmin Abdel-Magied, 27, an award-winning author and broadcaster, was scheduled to speak next week at the Pen World Voices Festival in New York. When she arrived in Minneapolis around 4 p.m., she said on Twitter, she was detained by border agents.

“I’m currently at the border, and they’ve said I’m being deported,” Ms. Abdel-Magied wrote on Twitter. “This should be fun. What are my rights?”

The agents, she said, told her they had canceled her visa and required her to return to London, where she currently lives.