President Donald Trump’s warning that protesters of Saturday’s US military parade in Washington, DC, will be met with “very heavy force” threatens to chill speech and underscores the need for protesters to take precautions around digital surveillance, privacy advocates tell The Verge.

A protest group has encouraged demonstrators to book up tickets to the military parade this weekend, in an effort to leave seats empty.

Trump has long admired the British royal family and his big birthday parade in Washington is the same day as the U.K.’s annual Trooping the Color.