Originally from here.

“Street preaching” (also known as “open-air preaching” and “open-airing”) is the public proclamation of a religious message. It is often accomplished with a voice louder than that used in conversation. For the purposes of this article/section, “street preaching” refers to the public proclamation of the law of God and/or the gospel of Jesus Christ, as well as the public reading of God’s Word. Other religions, such as Islam, employ “street preaching” as a method for sharing their religious message.

Christian street preachers find support for their craft in the Word of God where there are numerous examples of street preaching in both the Old and New Testament (this will be addressed in a later article). Additionally, Christian street preachers also draw biblical support for their craft in the definition of the New Testament Greek word translated “to preach.” The Greek word is kerusso. The word, which appears 60 times in the New Testament, is defined as follows:

“To be a herald, to officiate as a herald, to proclaim after the manner of a herald, always with the suggestion of formality, gravity and an authority which must be listened to and obeyed; to publish, proclaim openly: something which has been done; used of the public proclamation of the gospel and matters pertaining to it, made by John the Baptist, by Jesus, by the apostles and other Christian teachers.1”

Street preaching has a rich, post-apostolic age history. It has always been a part of Christianity, including the early church fathers, the pre-reformation teachers of the Middle Ages, the reformers, the Puritans, the pastors and circuit preachers of the Great Awakening, the revivalists of the early 20th century, and the heralds on the streets around the world today.