Johnson has pledged to take Britain out of the EU on Oct.
31 with or without a deal on future relations with the bloc.
The move to shut parliament for around a month in the period before that will hinder efforts by his opponents to stop him.
About 2,000 people gathered outside his office in Downing Street, chanting: "Liar Johnson, shame on you!" A sign read: "#StopTheCoup.
Defend our Democracy.
Save our future." The government says it is usual for parliament to be suspended before a new prime minister outlines his policy programme in a Queen's speech, now scheduled for Oct.
His supporters also say parliament usually breaks in late September, when the main political parties hold their annual conferences.
But his critics say the suspension, known as a prorogation, is unusually long and describe the move as a thinly veiled attempt to reduce the time that lawmakers will have to debate before Britain leaves the EU at the end of October.