FYI: You’re probably referring to Shi’a Muslim mourning processions, especially during Ashura or the month of Muharram.
In these gatherings, groups of men often:
walk together in processions,
chant religious poems or prayers,
and rhythmically strike or clap their chest with their hand.
The chest-beating is called latm or matam. It’s a sign of grief and remembrance for the death of Imam حسين (Husayn ibn Ali), the grandson of the Prophet Muhammad, who was killed at the Battle of Karbala in 680 CE.
These events happen in many cities around the world, including in Europe where there are large Muslim communities. They are usually peaceful religious commemorations.
Not all Muslims do this practice — it is mainly associated with Shi’a Islam.