When Linda Sarsour, a New York community organizer, saw on social media that the Islamic Center of San Diego had been attacked on Monday, she immediately texted Imam Taha Hassane.

Ms. Sarsour, one of four national co-chairs of the Women’s March on Washington in 2017, has crisscrossed the country speaking at mosques and community centers. Shortly after the shooting, which killed three people, she said that the Islamic Center of San Diego was “a model for the rest of the country.”

“It’s such a wonderful place, and I’m not even saying that to you just because it’s this mosque,” she said in an interview. “I’ve been there so many times.”

The mosque’s website says that its mission is to “work with the larger community to serve the less fortunate, to educate, and to better our nation.” It holds prayers five times a day, as well as a Friday sermon.

Ms. Sarsour said the mosque’s diversity is its most striking aspect.

“People from African Americans to African immigrants to Arabs to white converts to Eastern Europeans to Latinos,” she said. “It’s literally the most diverse mosque that I’ve ever been to in my whole life.”

The center, she said, does a lot of community service and mobilizes parishioners to go to pro-immigrant rallies. It hosts open houses for the general public.

“Their sermons are all in English because of the diversity of the mosque,” she said. “You can’t do it in Urdu, or Arabic, because of that diversity.”

Ms. Sarsour said that it was more than just a place to pray, and that there was always an activity. At the time of the shooting, parents would have been in the lounge area after dropping their children at the adjacent Islamic school, she said.

She said that the imam, Mr. Hassane, was active in the larger community and part of many national interfaith efforts.

“He’s not like a regular imam, like the ones who go to the mosque, lead a prayer and go home,” she said.