The developer of an Islamic cultural center that opened Wednesday evening near the site of the terrorist attacks that leveled the World Trade Center says the biggest error on the project was not involving the families of 9/11 victims from the start.

People crowded into the center, where a small orchestra played traditional Middle Eastern instruments and a photo exhibit of New York children of different ethnicities lined the walls. The enthusiasm at the opening belied its troubled beginnings.

"We made incredible mistakes," Sharif El-Gamal told The Associated Press in an earlier interview at his Manhattan office.

The building at 51 Park Place, two blocks from the World Trade Center site, includes a Muslim prayer space that has been open for two years. El-Gamal said the overall center is modeled after the Jewish Community Center on Manhattan's Upper West Side, where he lives.

"I wanted my daughter to learn how to swim, so I took her to the JCC," said the Brooklyn-born Muslim. "And when I walked in, I said, 'Wow. This is great.'"

The project has drawn criticism from opponents who say they don't want a mosque near the site of the Sept. 11 terrorist attacks.

The center is open to all faiths and will include a 9/11 memorial, El-Gamal said. He called opposition to the center — which prompted one of the most virulent national discussions about Islam and freedom of speech and religion since Sept. 11 — part of a "campaign against Muslims."

Last year, street clashes in view of the trade center site pitted supporters against opponents of the center.

When the center was first envisioned several years ago, activist Daisy Khan and her husband, Imam Feisal Abdul Rauf, played a major, vocal role. But they soon left the project because of differences with the developer.

El-Gamal, 38, confirmed Wednesday that they parted ways because "we had a different vision." He declined to elaborate.

The couple said they had discussed plans for Park51, as the center is known, with relatives of 9/11 victims, first responders and others, including the possibility that it could become a multifaith center focusing on religious conflict. But El-Gamal wishes victims' families had been involved earlier — before the center became a point of contention.

"The biggest mistake we made was not to include 9/11 families," El-Gamal said, noting that the center's advisory board now includes at least one 9/11 family member.

At first, "we didn't understand that we had a responsibility to discuss our private project with family members that lost loved ones," he said, and they did not "really connect" with community leaders and activists.

But today, "we're very committed to having them involved in our project. ... We're really listening," he said.

Pointing to the inclusivity of a center that critics feared would be polarizing, El-Gamal noted that the featured photographer in the "NYChildren" exhibit is Danny Goldfield, who is Jewish.

The Brooklyn photographer was inspired to create the exhibit by the story of Rana Sodhi, a Sikh who emigrated from India and settled in Arizona. His brother Balbir was killed in a retaliatory hate crime four days after Sept. 11.

Sodhi made the trip to New York for the opening and wore a tie decorated with heart-shaped American flags. He still runs the gas station where his brother was killed.

"My heart is so warm when I hear Danny is doing this exhibition in Park51," Sodhi said.

Goldfield said he has photographed children with roots in 169 countries since 2004. He hopes to find subjects representing 24 other countries to complete the project. Some of the photographs had been exhibited elsewhere, but the opening marked the first time all were shown together.

He said there was a synergy between the themes and spirit of his project and those of the center, particularly with regard to community participation and openness.

"They want to build a center for everyone that's represented on the walls here," he said.

Recalling the controversy over the center, he said he didn't want to pass judgment on its opponents. But he said he'd like them to see the show "more than anyone."

Afsana Khundkar, a native of Afghanistan whose 12-year-old son, Waseem, was one of the children photographed for the exhibit, said her family was honored to participate in the project.

"It's promoting good things in the world," she said. "The most important thing is to involve the children in the good things."

The space had been cleared out and the walls painted a stark white for the exhibit. The renovations were funded with $70,000 raised on the website Kickstarter. The modest first-floor space is intended to function as a temporary center until groundbreaking on an entirely new building.

El-Gamal told the AP that fundraising is under way to complete a 15-story building that will also include an auditorium, educational programs, a pool, a restaurant and culinary school, child care services, a sports facility, a wellness center and artist studios.

The mosque is especially needed in lower Manhattan, he said, because thousands of Muslims either work or live in the neighborhood, "and in our religion, we must pray five times a day."

At the opening, an ebullient El-Gamal told reporters the project had been framed by others throughout the debate over its existence.

"Today, for the first time, everyone gets a little bit of a glimpse into the future of what Park51 is going to offer New York," he said.