The suit was filed two days after city councilors passed the ban on the sale and possession of assault weapons.
The ban was proposed in response to the high school shooting in Parkland, Florida.
Democrat state Rep. Alec Garnett proposed the bipartisan 'gun restraining order' bill, which empower judges to temporarily remove firearms from potentially dangerous people.
After weeks of debate, the city council will again take up a proposal to bar assault-style weapons, high-capacity magazines and bump stocks.
There were many parallels drawn between the 1999 Colorado shooting and the February shooting in Parkland, Florida.
Why do high schools allow the NRA to sponsor groups? Why can't gun violence research get more funding?
“Enough is enough!” chanted the protesters around the state. “Not one more!” "Prayers are not enough!”
Arapahoe High School junior Paul Gordon is helping organize one such walkout. Golden High School sophomore Enzo Perri is staying put.
The school shooting in Florida last week in which 17 people died broke Avery Griggs’ heart.
A group of teens held a similar "lie-in" at the start of the week outside of the White House to demand tougher gun laws.