Why not a eucharistic anti-gun event?
A number of activist organizations are declaring March 15-17 "National Gun Violence Prevention Sabbath Weekend."
It's not clear if this is meant to replace The Brady Center's "God Not Guns Sabbath," which has been observed on the last weekend of September for a number of years. But the organizers seem eager to keep the event broadly ecumenical and interfaith. So if you look through the resources provided, you'll find things for Jewish congregations and a variety of Christian traditions. There are even some secular statements and a quote from Thich Nhat Hahn that would be right at home in a Unitarian-Universalist worship service. I didn't see anything geared specifically to Muslims, but perhaps I overlooked it.
This broad approach seems sensible for an event aimed at large participation. But I do have to wonder if the organizers aren't missing an opportunity rooted in the specificity of Christian worship. It seems to me that eucharistic worship offers exactly the kind of witness needed in this situation. I am following here the thought of William Cavanaugh, who finds in the eucharist a "different imagination" and a "dangerous memory" that allows a new society to be built right under the noses of principalities and powers: