CAMBRIDGE — More than three years after he was barred from entering the United States, South African political science professor Adam Habib finally got to speak to audiences around Boston this week. And he seized the platform to call on President Obama to declare that “ideological exclusion is wrong.’’

Adam Habib, a South African scholar and opponent of the Iraq war who was denied entry to the United States under a provision of the Patriot Act, spoke at Harvard Law School on Wednesday. (Christopher Ott/Aclu of Massachusetts) In January, Secretary of State Hillary Rodham Clinton withdrew the ban on a visa for Habib, without explaining why. Several Boston groups, including the Massachusetts branch of the American Civil Liberties Union, had brought a lawsuit in 2007 challenging the denial of visas to Habib, his wife, Fatima, and their two children. The case argued that Habib, an outspoken critic of the Iraq war, was being excluded from the country on ideological grounds.

Addressing a small group at Harvard Law School on Wednesday, Habib did not sound like the terror-stained radical the Bush administration suggested he was in October 2006, when Department of Homeland Security officials turned him back at Kennedy Airport in New York.

He sounded more like a man making a sentimental homecoming to a country where he lived for three years and earned his doctoral degree.

“For me, the United States is a second home,’’ Habib said. “Home is where memories are made, and other than South Africa, the place where I’ve lived the longest is the United States. It’s the place where I have friends; where Fatima and I love the excellent dumplings in Chinatown; the place where my son was conceived; where we fed the ducks in Central Park; where I took my son to Disney World. It’s the place where I have memories. I should not be denied access to the place where those memories were made by the arbitrary act of a government official because they didn’t like what I had to say.’’

Melissa Goodman, an ACLU attorney who argued Habib’s case, said Habib is among dozens of academics, artists, and activists excluded from the United States with no explanation after Sept. 11, 2001, under a provision of the Patriot Act.

She said she examined the writings of Habib, who had come to the United States in 2006 on behalf of a South African research council to meet with groups including the World Bank and the Brookings Institution, and looked through his record, but found nothing to justify his exclusion.

“I very confidently decided that the government was crazy, that there was no legitimate reason to bar him on terrorism grounds,’’ she said.

The American interest at stake, Goodman said, goes beyond protecting Habib’s civil rights.

“We see this as a form of censorship,’’ she said. “It actually prevents people like you from hearing diverse views. It is imposing an ideological litmus test at the border.’’

State Department spokesman Darby Holladay said yesterday in an e-mail that while the government would not disclose the reasons for the original ban, “Both the president and the secretary of state have made it clear that the US government is pursuing a new relationship with Muslim communities based on mutual interest and mutual respect.’’

Habib said that when he arrived in Washington last week, his treatment was decidedly different from the night in 2006 when two armed guards escorted him back onto a plane to Johannesburg. This time, he said, he was met by immigration officials, who whisked him through customs, collected his suitcases, and provided a car to drive him and his wife to their hotel.

“So I went from one extreme to the other,’’ Habib said. “I felt like a rock star.’’

Habib, deputy vice chancellor at the University of Johannesburg, met with academic groups yesterday at the Massachusetts Institute of Technology and Boston University. He heads to New York today.

He told the Harvard audience that he suspected he was barred entry because of his very public opposition to the Iraq war. Eventually, he said with a smile, he came to conclude that “like all bureaucracies, they had worked out some mathematical formula: Opposition to the war, two points; Habib — Muslim family name — one point.’’

Habib said the cost of US willingness to exclude people on ideological grounds is that other countries with fewer avenues for legal challenge then feel they can adopt similar harsh measures against dissident voices.

“When the US acts in this way, the ripple effects across the globe are dramatic,’’ Habib said. “Antiterrorism legislation is now flowing freely across all countries.’’

With human rights violations becoming more transnational, civil rights defenders such as the ACLU, Amnesty International, and Human Rights Watch have to work together more closely against them, Habib said.

“When Iranian scholars are detained, all of us have to be heard. In Iran, in Zimbabwe, in Myanmar, unless there are global pressures and collective solidarity, those struggles will never be successful.’’