They went seven times around the Planned Parenthood centre, which undertakes a number of procedures including abortions.
Archbishop Samuel Aquila of Denver described leading the event as "truly a moment of grace, a moment of blessing."
The procession surrounded the facility with silent prayer around an hour.
Speaking to the archdiocesan newspaper, Denver Catholic, Archbishop Aquila said it was a "moment of praying to our Lord that hearts may be changed. It was wonderful to see how many turned out today."
The archbishop announced his intention to lead the procession in February.
Karna Swanson, communications director for the archdiocese said: "We were honestly expecting 500 to 800 people. Three times that number showed up.
"This provided a bit of a challenge for us logistically, as 1,800 people don't exactly fit on the sidewalk of a city block."
The Archdiocese of Denver website invited people to attend, stating: "No shouting or arguing. Only prayerful witness to the love and mercy of God."
Before the procession started, Fr Scott Bailey, secretary to Archbishop Aquila, stressed the importance of silence to the crowd: "Silence is an essential part of the procession as we unite our voices with those who have been silenced by abortion."
Planned Parenthood says it is a trusted health care provider and an informed educator which delivers vital reproductive health care, sex education, and information to millions of women, men, and young people worldwide.
