Security guards at major San Francisco tech offices say they plan to strike starting Friday after months of contract talks over pay and benefits.

The walkout involves guards represented by Service Employees International Union-United Service Workers West. The union says negotiations have been underway with 15 private-security contractors and employers, and it accuses companies of unfair labor practices during the bargaining process.

According to the union, the current offer would raise pay by 65 cents over the next two years. Guards now earn a median hourly wage of $20.09, according to a UC Berkeley study cited in the dispute, while the union says it is seeking a path to $30 an hour for all officers.

One major contractor, Allied Universal, said it does not comment on ongoing labor negotiations. The guards also marched through downtown San Francisco this week as the labor fight intensified.