Seattle officials voted unanimously on Monday to increase the city's minimum wage to $15 an hour, the highest rate in the country.

"A year ago, $15 was just a number on fast food strikers' picket signs," Working Washington, a labor group that has long supported the increase, said in a statement. "Today it's set to become reality for 100,000 Seattle workers."

City Councilwoman Kshama Sawant, an avowed socialist, celebrated the decision: "We did it. Workers did this. ... We need to continue to build an even more powerful movement."

The measure was first proposed by Democratic Mayor Ed Murray, who was expected to sign it into law on Tuesday. The increase will be imposed gradually over the course of several years, first on larger businesses that employ more than 500 employees by 2017 and then on all businesses by 2021.

The first increases will start for smaller businesses at $10 while larger businesses will start at $11.

"Seattle, and other cities, are taking direct action to close our nation's huge income gap because the federal and state governments have failed to do so," City Councilman Nick Licata said. "By significantly raising the minimum wage, Seattle's prosperity will be shared by more people and create a sustainable model for continued growth."

The approved measure is the result of a compromise between business leaders and advocates of higher wages, a spokesman for Murray said, adding that the leaders from nonprofit groups also played a role.

However, opponents of the measure claim that it unfairly targets certain franchises that have been classified as “larger” while other businesses are exempt, CNN Money reported.

"The City Council's action today is unfair, discriminatory and a deliberate attempt to achieve a political agenda at the expense of small franchise business owners," the International Franchise Association, which has filed a discrimination lawsuit against the city, said in a statement.

Meanwhile, certain wage advocates say that the proposed measure does not go far enough.

An activist group called 15 Now has already launched a campaign to collect signatures for a ballot measure that would immediately impose a $15 per hour rate on larger businesses and speed up the process of imposing the wage hike on business with fewer than 250 full-time employees.

At $9.32 an hour, Washington already has the highest statewide minimum wage in the country, well above the federal minimum rate of $7.25.

Washington is followed by Oregon, Vermont, Connecticut and New Jersey in holding the highest rates in the country, at $9.10, $8.73, $8.70 and $8.25 per hour, respectively.

The move by Seattle's city council comes at a time when the White House and congressional Democrats have battled Republican lawmakers over efforts to increase the federal minimum wage to $10.10 an hour.

Meanwhile, major companies, including McDonald's, Panera and Costco, announced recently that they plan to install computer kiosks at several locations, a move that is intended to reduce the annual cost of labor.