Federal agents and local police officers conducted several raids at homes and businesses around Los Angeles on Wednesday in a drug enforcement operation that the authorities said was targeting a network of fentanyl and methamphetamine dealers concentrated in the MacArthur Park area near downtown.

The operation resulted in 18 arrests, including two people believed to be the main source of fentanyl and methamphetamine in the park, officials said. Seven other people are being sought. A federal complaint filed Wednesday charges 25 people with possession of a controlled substance with the intent to distribute and distribution of a controlled substance.

The historic 35-acre park, which includes a lake, was once an idyllic haven amid an urban landscape in the Westlake neighborhood of Los Angeles, just west of downtown. But it has been troubled in recent years by homelessness and drug and gang activity even as city officials have taken steps to address the issues.

News and police helicopters whirred overhead during an outdoor news conference next to the park as Bill Essayli, the first assistant U.S. attorney for the Central District of California, was joined by Jim McDonnell, the Los Angeles police chief, and Anthony Chrysanthis, the special agent in charge of the Drug Enforcement Administration’s Los Angeles field office, in announcing the monthslong investigation that led to the arrests on Wednesday.

“Today’s operation is only one step taken by a handful of agencies working hard to alleviate anguish and the sense of hopelessness plaguing MacArthur Park,” Mr. Chrysanthis said.

Images captured by news helicopters showed D.E.A. agents in tactical gear raiding stores along a busy commercial corridor across the street from the park. Agents served search warrants at six businesses there, Mr. Chrysanthis said. More than 100 Los Angeles police officers participated in the operation, he added.

In court filings, prosecutors accused Mallaly Moreno-Lopez, 31, and her boyfriend, Jackson Tarfur, 28, of Los Angeles, of hand-delivering drugs to the businesses near the park for stashing and later distribution to street dealers. The authorities began their investigation in March, according to court filings.

If convicted, Ms. Moreno-Lopez and Mr. Tarfur face minimum 10-year prison sentences, officials said.

The authorities also raided the couple’s home in South Los Angeles, and the home of Yolanda Iriarte-Avila, 40, who court filings said had supplied Ms. Moreno-Lopez with methamphetamine. Officials said they had seized 40 pounds of fentanyl at Ms. Iriarte-Avila’s home in Calabasas on Wednesday.

The operation at MacArthur Park highlighted larger tensions between the federal government and California’s political leaders, whom the Trump administration has long accused of fostering lawlessness.

Last summer, the city was the scene of a federal show of force led by Gregory Bovino, the Border Patrol commander-at-large who was subsequently ousted from the agency. In July, Mr. Bovino led heavily armed immigration agents and National Guard troops in a march across the park in an operation that led to a confrontation between Mr. Bovino and Karen Bass, the mayor of Los Angeles.

At the news conference, Mr. Essayli, an ally of President Trump, was joined by Dr. Mehmet Oz, the former talk show host who now runs the Centers for Medicare and Medicaid Services and has recently embarked on a campaign targeting hospice care fraud in the state.

Both men blamed state and local policies for the conditions at the park. Mr. Essayli opened the news conference by saying, “We’re here today because California policies have failed.”

Mayor Bass and city officials have tried in recent years to clean up the park, exploring the installation of a fence and ramping up the police presence in the area.

“As we work to change MacArthur Park so that it’s safe and clean for families, we have zero tolerance for people who deal deadly drugs and prey on the community,” Mayor Bass said in a statement. “We will continue to aggressively pursue our comprehensive strategy to restore MacArthur Park.”

The mayor, who is running for re-election, also acknowledged the operation when asked about it at a Wednesday evening mayoral debate, and pointed to her plan to hire more police officers.