Not long after noon on Wednesday, Dan Rivera climbed into his Ford Explorer and pulled out of the parking lot next to Lawrence City Hall.

Rivera is the mayor of Lawrence. Some mayors have cops as drivers, but Lawrence needs every available cop on the street. Rivera drives himself.

“Let’s go to the bridge,’’ he said, almost to himself. “Under the bridge.’’

The Casey Bridge spans the Merrimack River in the old mill city. Under the Casey, there is an encampment of Dickensian sprawl and squalor where the homeless, the addicted, the mentally ill, the forgotten seek shelter.

A huge pile of trash bags and abandoned furniture sat near the entrance.

“We’ve got anywhere from 60 to 100 people living there,’’ Rivera said. “Our homeless population has quadrupled.’’

Many of the people under the bridge are addicted to heroin. So are many of the countless panhandlers who shuffle around the city all day, every day.

“We have food pantries, we have homeless shelters, but we’re not equipped for this,’’ he said, steering the Explorer up and away from under the bridge.

Last month, Rivera went to three high school graduation receptions at which parents accepted diplomas for kids who died from overdoses.

Rivera pointed to a McDonald’s where they had to hire an armed guard.

“The panhandlers were going up to people at the drive-through,’’ Rivera said.

At the busy intersection of Merrimack and Parker, addicts had jammed the crossing buttons so traffic would stop more frequently.

On Merrimack Street, a young man handed a small bag of white powder to an older woman. They didn’t even try to hide it.

“We’ve spent $780,000 to fix up 22 parks,’’ Rivera said. “We’ve got needles in every one of them.’’

On West Street, where workers are still rebuilding the park, a few young men, their faces tanned but left lifeless by drugs, eyed us warily and headed to the rear of the park.

Manny Baez runs an auto repair shop next to the park.

“I’ve seen 25, 30 cars with New Hampshire plates drive by today,’’ Baez said.

The drivers were looking for heroin, Baez said.

“This neighborhood matters,’’ Rivera said, pointing to a successful charter school a little further down West Street. “That school sends 100 percent of its graduates onto college. Some of those kids walk to school every day and see this stuff.’’

Rivera says the Lawrence police arrest plenty of heroin dealers. He said the Lawrence-based task force led by the State Police has done a terrific job intercepting shipments from New York. But while law enforcement is essential, Rivera says it’s not enough. It can’t be just supply. It’s got to be demand, too.

“We need treatment, and it has to be compulsory,’’ he said. “And it has to be for a long time.’’

The problem doesn’t begin and end in Lawrence. People come to Lawrence to cop drugs, then they drive home or walk under the bridge.

The heroin problem, worsened by fentanyl, is bigger than any struggling city, any state. It’s a national problem and it needs national solutions. Rivera is desperate enough to think big.

He says why not use Fort Devens? Or take a closed hospital out of mothballs and create 1,000 beds?

“We need free or affordable, immediate care, for six to nine months. But treatment has to be compulsory. The state and federal governments have to step up. Communities like Lawrence can’t be expected to do this alone,’’ he said.

Rivera pulled into the City Hall lot and looked at his phone. He had a dozen e-mails, all about House Speaker Paul Ryan’s visit Thursday to the New Balance plant in Lawrence to talk about tax reform.

Just before Dan Rivera was able to mention his ideas about compulsory treatment and more federal help to Ryan, a huge fire broke out and he had to excuse himself and rush to the scene. In Lawrence, there’s always some fire to put out.

Kevin Cullen is a Globe columnist. He can be reached at cullen@globe.com