Electricians and other construction workers taking part in a national day of action marched from the Shard site at London Bridge to Blackfriars Station on Wednesday lunchtime.
The protesters say that Britain's large electrical contractors have pulled out of national agreements and many skilled electricians will see their pay cut.
Employers deny that electricians will be worse off as a result of the changes.
Wednesday's day of action at some of the country's largest construction sites started at the Pinnacle site in the City of London, moving to the Shard before a march along Southwark Street to Blackfriars Bridge.
The event was due to culminate in a rally outside Parliament.