Haitian President Michel Martelly was attacked as he walked in a Carnival procession in downtown Port-au-Prince, but he escaped unharmed, the National Palace said Friday.
The statement said only that "troublemakers" who had taken refuge in a university were responsible for the afternoon attack on Martelly near the Champs de Mars plaza.
Witnesses told The Associated Press that rocks were thrown at the president and his accompanying motorcade from behind the University of Haiti. Police responded by firing shots in the air and unleashing tear gas.
One witness said he saw Martelly hit in the shoulder by a rock but otherwise wasn't injured.
Martelly "almost fell," said Stanley Lazard, 25. "Two security guards embraced him and put him in the car. He definitely got hit with a rock."
The presidential office and witnesses both said Martelly was taken without injury to the nearby National Palace. Martelly left the National Palace about an hour later and went on a walk under a heavy police guard, before returning to the presidential residence.
The attack on the president was the second since he was inaugurated last May, and pointed to growing tension between Martelly and his critics.
Susan Rice, the U.S. ambassador to the United Nations, said this week on behalf of a visiting U.N. Security Council delegation that Martelly and Parliament need to "rise above their interests and work together in the spirit of compromise to overcome their common challenges."
The remarks came a week after anti-Martelly protesters demanded he heed lawmakers' request that prove he's eligible to hold office by showing his travel documents. Several senators suspect Martelly holds dual citizenship, which would bar him from office under Haiti's constitution. Martelly said he's under no obligation to show the paperwork.
The episode on Friday led to clashes between students and Martelly supporters, who tore down a fence and entered the school grounds. Students said the president's backers smashed windows of cars that were parked in the courtyard of the university.
"The people with the president, they came inside and started throwing stones at the students," said sociology student Frantz Legros, 22. "They came to the front to beat the students."
Another man, Pythagone Dumais, showed reporters a bloody hand and said he was attacked by a Martelly supporter. He said the attacker grabbed a beer bottle from him, smashed it and slashed his hand.
The Haitian Red Cross sent a bus to safely evacuate about a dozen university students from Canada, France and Brazil. They were attending a conference with Haitian students.
Haitian police spokesman Gary Desrosiers said he couldn't comment.
Government prosecutor Jean-Renel Senatus showed up at the university Friday evening to begin an investigation as he looked at the damaged vehicles but declined to comment.
The first attack on the president came two months after he was inaugurated. Protesters lobbed rocks and soda bottles at him and his entourage as they walked through a street in Cap-Haitien, Haiti's second largest city. Martelly wasn't injured.
The public plaza where Martelly was attacked Friday has been home to nearly 20,000 people who were displaced by the 2010 earthquake.
The government and several aid groups began on Friday to relocate the residents through rental subsidies. The Canadian government is providing $19.9 million over two years to finance the resettlement effort.