The United States' tallest building, the iconic Sears Tower, became officially known as Willis Tower on Thursday amid opposition from thousands of nostalgic Chicagoans and Americans.

Chicago Mayor Richard Daley and Willis Group Holdings chairman Joseph Plumeri unveiled the new sign of the iconic 110-story tower during a renaming ceremony Thursday.

The British insurance firm that bought the building also bought the right to change its name.

Plumeri shrugged off protests from a Facebook group that launched a signature campaign to oppose the name change and defended the renaming of Sears Tower by highlighting his company's contributions to Chicago, including bringing 500 jobs to the building, a $17 million transfer to the building, and a $100,000 pledge for the city's hosting of the 2016 Olympics. Willis Group will occupy three floors of the building for 15 years.

"Having our name associated with Chicago's most iconic structure underscores our commitment to this great city, and recognizes Chicago's importance as a major financial hub and international business center," Plumeri said in a press release.

Sears Tower opened in 1973 as the headquarters of Sears, Roebuck and Co. That company vacated the building in 1993 but the new owner, Hoffman Estates, retained the name.