As Libyan anti-government forces massed in a city outside Tripoli ready for an expected offensive, U.S. President Barack Obama called on Muammar Gaddafi to leave power immediately, saying that after his violent crackdown he had lost the legitimacy to rule. An Associated Press reporter who reached Zawiya, 30 miles west of Tripoli, confirmed Sunday that anti-Gaddafi rebels were in control of the city center. They had deployed army tanks and anti-aircraft guns mounted on pickup trucks, according to the reporter, but were surrounded by pro-Gaddafi forces, also backed by tanks and anti-aircraft guns. By all reports, Gaddafi still holds the most of Tripoli, which is home to 2 million of Libya's 6.5 million people. But Zawiya, a town of 200,000 close to an oil port and refineries, is the nearest population center to Tripoli to fall into the opposition hands. Police stations and government offices inside the city have been torched and anti-Gaddafi graffiti was everywhere. Many buildings are pockmarked by bullets. “Listen, nobody is leaving this country. We live here, we die here.” ~Seif al-Islam GaddafiGaddafi has launched by far the bloodiest crackdown in a wave of anti-regime uprisings sweeping the Arab world. The U.N. Security Council imposed sanctions on Libya and issued a travel ban on Gaddafi, his family and his officials over the weekend. The United States and Britain imposed their own sanctions, too. The White House said Sunday that in a conversation with German Chancellor Angela Merkel, Obama stated “that when a leader's only means of staying in power is to use mass violence against his own people, he has lost the legitimacy to rule and needs to do what is right for his country by leaving now." Until Saturday, U.S. officials had held back from openly backing the protest movement, insisting that it was for the Libyan people to determine whether or not Gaddafi should leave office and the country. Meanwhile, Gaddafi's son rejected the president's call and repeated assertions he made Friday that the number of deaths at the hands of pro-Gaddafi forces during the 10-day uprising had been exaggerated. "Listen, nobody is leaving this country," said Seif al-Islam Gaddafi in an interview with ABC's Christiane Amanpour. "We live here, we die here." When Amanpour asked him specifically about Obama's call for a new Libyan government, the younger Gaddafi said: "It's not an American business, that's number one. Second, do they think this is a solution? Of course not." There is a "big big gap between reality and the media reports" Gaddafi, once seen as a successor to Muammar, told Amanpour. "The whole south is calm. The west is calm. The middle is calm. Even part of the east." However, GlobalPost's Nichole Sobecki reported Saturday from Benghazi, Libya's second largest city, that opposition forces were in full control and had been largely successful in maintaining law and order. Volunteers had taken over the former internal security building and courthouse, organizing the town’s security, cleaning up, providing food, assisting foreign journalists and beginning the long road toward good governance, Sobecki wrote. The only gunfire on the streets was in celebration, uniformed traffic police direct traffic in the center of town and a few of the city’s banks had reopened. "For 42 years we were kept alive-dead," Hana Kahlig, a human rights organizer volunteering at the courthouse, told Sobecki. "We didn't have freedom of speech, education is below zero. He [Gaddafi] treated us like cockroaches, forcing us to live without lights or water. Now we want to live with freedom, just like everyone else." In Zawiya on Sunday, chants of "Gaddafi Out," and "Free, Free Libya," were heard, according to the AP. Graffiti read: "Down with Gadhafi, the mass murderer," and an effigy of Gaddafi hung from a light pole in the city's main square with the words "Execute Gadhafi" emblazoned on its chest. Before Zawiya fell to rebel forces, Gaddafi scolded the city residents on Thursday, accusing Osama bin Laden and Al Qaeda of using hallucinogenic drugs to incite unrest. "Shame on you, people of Zawiya. Control your children," he reportedly said. "They are loyal to bin Laden." And: "What do you have to do with bin Laden, people of Zawiya? They are exploiting young people ... I insist it is bin Laden."