On a warm morning earlier this month, about 600 Los Angeles police officers gathered in the empty parking lot at Dodger Stadium for some high-stakes role playing.

Most pretended to be protesters -- standing in for the ones expected to converge on downtown Los Angeles on Thursday as part of May Day immigration rallies planned across the country.

As some in the mock crowd threw bottles and acted the part of agitators, officers assigned to undercover "extraction units" quickly and quietly isolated the rabble-rousers and hauled them away.

"Is everyone clear on chain of command?" Michael Hillmann, a deputy chief in the Los Angeles Police Department, asked afterward. "Everyone clear on who is in charge of what?"

Under normal circumstances, such questions -- and the elaborate exercise -- might be considered overkill. But on the heels of last year's disastrous May Day, when police injured marchers and journalists during a botched effort to clear MacArthur Park, LAPD leaders are not in the mood to leave things to chance.

The debacle was a setback in the department's effort to improve its image in the city and shed a reputation for unwarranted aggression.

For Police Chief William J. Bratton, the incident was an embarrassment and one of the most serious tests of his leadership since he became chief in 2002. His decision to quickly and publicly apologize for his officers' handling of the protesters managed to temper widespread outrage somewhat, but it also irked police union leaders, who accused him of jumping to conclusions.

"It was probably the most significant multiple set of crises all occurring at the same time that I had ever faced in my career," he said in a recent interview. "But in responding to them, it was always with the focus of 'OK, how out of this negative can we get something positive?' "

In recent months, the department has been planning for this May Day's event: gathering intelligence, meeting with organizers and training officers in contingency plans and crowd control.

"Last year, it just wasn't organized. It was a disaster," Hillmann said. "It was as if the people involved went into it with the idea that the event would work itself out. Crowds do not manage themselves."

Organizers of Thursday's march, which is expected to attract from 20,000 to 100,000 people, have voiced cautious optimism that this year's event would go smoothly. They have commended Hillmann and other LAPD leaders for their efforts.

"It's up to the LAPD to follow through on their promise, to be there to support the march and make sure all of the march participants have a good experience," said Bethany Leal of the Multi-Ethnic Immigrant Worker Organizing Network, one of the sponsors of the march and rally.

Juan Jose Gutierrez of Latino Movement USA echoed Leal, adding that many immigrants still distrust police because of their heavy-handed actions last year.

"There has been progress, but I don't think the relationship has been totally repaired," he said. "A lot of us are in a wait-and-see mode."

The scrutiny will extend far beyond the city's Latino communities. Federal monitors, who oversee the department's efforts to comply with a set of mandated reforms imposed after a corruption and abuse scandal in the late 1990s, will be on hand to observe.

Members of the department's civilian oversight commission, who have sat in on some of the planning meetings, and legal observers from several community and civil rights groups also will be on the streets.

The training at Dodger Stadium arose directly out of what went wrong last year. Near the end of a largely peaceful day of immigrant rights rallies, a group of 20 to 30 people at MacArthur Park provoked police by throwing sticks and water bottles filled with ice and gravel.

Police failed to effectively cut off the violent pocket from the rest of the crowd and, amid growing confusion, commanders gave an order to disperse the entire gathering. The message to leave was broadcast but only in English and from a speaker on a noisy helicopter.

Chaos ensued as officers in riot gear pushed their way through the park, wielding batons and firing nonlethal bullets.

More than 240 protesters and journalists have claimed they were injured, as well as 18 officers.