Across the street in the House of Delegates building, some 950 people both in favor and opposed signed up to testify on the bill.
A version of the governor's bill with more than 40 amendments passed in the Maryland Senate on Thursday.
Those attending the rally heard speeches from members of the clergy, a hunter and women who spoke “as mothers.”
O’Malley and Lt. Gov. Anthony G. Brown (D) also spoke at the rally.
“We can make this world a kinder and more loving and safer world,” O’Malley said. “We are here to take the tears, to transform the unspeakable loss of a person standing at the grave of a child and turn it into action.”
“Enough is enough,” Brown said. “Today, we are standing with the 85 percent of Marylanders who support a gun licensing plan.”
Many of those in the crowd said they were there on behalf of friends or relatives who were victims of gun violence.
Joanna White, a revered from St. Paul’s Church in Frederick, Md., said she was there on behalf of “a friend of mine, a priest, she was murdered in her office doing what we do every day, helping people from off the street. This man, she had helped him before but he was mentally ill and he had a gun.”
“There are certain people who shouldn’t have guns,” she said. “And no one needs an assault weapon.”
Edwin Green, 36, pulled his three children out of school for the day so they could attend the rally. “We’re here in support of common-sense gun control,” he said, “I have an obituary here of my cousin who got killed by a person that was mentally ill. It was a case of mistaken identity, up in Baltimore, there was gang violence and they thought he was someone else. Shot him right in the middle of the day. If they had had stricter laws, maybe he would still be alive.”
Aaron C. Davis contributed to this report.