city for working-class people. The city population shrank by nearly 700,000. The City Council devised "Plan 21" to improve neighborhoods and focused on creating "Suburbs within the city" near downtown and the lakefront. It built public housing to try to improve housing standards in the city. As a result, many poor were uprooted from newly created enclaves of Black, Latino, and poor people in neighborhoods such as Near North, Wicker Park, Lakeview, Uptown, Cabrini–Green, West Town and Lincoln Park. The passage of civil rights laws in the 1960s also affected Chicago and other northern cities. In the 1960s and the 1970s, many middle- and upper-class Americans continued to move from the city for better housing and schools in the suburbs. Office building resumed in the 1960s. When completed in 1974, the Sears Tower, now known as the Willis Tower, was at 1451 feet the world's tallest building. It was designed by the famous Chicago firm of Skidmore, Owings & Merrill, which designed many of the city's other famous buildings. Mayor Richard J. Daley served 1955–1976, dominating the city's machine politics by his control of the Cook County Democratic Central Committee, which selected party nominees, who were usually elected in the Democratic stronghold. Daley took credit for building four major expressways focused on the Loop, and city-owned O'Hare Airport (which became the world's busiest airport, displacing Midway Airport's prior claims). Several neighborhoods near downtown and the lakefront were gentrified and transformed into "suburbs within the city". He held office during the unrest of the 1960s, some of which was provoked by the police department's discriminatory practices. In the Lincoln Park, Lakeview, Wicker Park and Humboldt Park communities, the Young Lords under the leadership of Jose Cha Cha Jimenez marched and held sit ins to protest the displacement of Latinos and the poor. After the assassination of Martin Luther King Jr. in 1968, major riots of despair resulted in the burning down of sections of the black neighborhoods of the South and West sides. Protests against the Vietnam War at the 1968 Democratic National Convention, held in Chicago, resulted in street violence, with televised broadcasts of the Chicago police's beating of unarmed protesters. In 1979, Jane Byrne, the city's first woman mayor, was elected, winning the Democratic primary due to a citywide outrage about the ineffective snow removal across the city. In 1983, Harold Washington became the first black mayor of Chicago.