Two Los Angeles police officers are under investigation for allegedly using blackmail and intimidation to force women to have sex with them while on duty.

Luis Valenzuela and James Nichols, veteran LAPD officers in the Hollywood division, are accused of targeting at least four women, in some cases taking them into an unmarked car to secluded areas. The accusations, if confirmed, will taint the image of a force that has shed much of its reputation for thuggery since the Rodney King riots of 1992.

The LAPD chief, Charlie Beck, said on Thursday that he was saddened by the allegations and that investigations were continuing. "If they are true, it would be horrific," he said.

Detectives from the department's internal affairs unit suspect the officers preyed on women whom they had arrested previously or who worked for them as informants, according to a search warrant reviewed by the Los Angeles Times. The pair lured victims into their car and used the threat of jail to coerce sex, the warrant alleges. Four women have made independent accusations.

Detectives intended to confront the officers next week but rushed to do so this week, after one of the women filed a lawsuit. The detectives sequestered the officers and seized their computers and phones. Valenzuela and Nichols were expected to remain off duty pending the investigation's outcome.

The first accusation was made in January 2010, when a woman who worked as a police informant told a narcotics unit supervisor that the officers, wearing plain clothes, had lured her into a Volkswagen Jetta. One allegedly exposed himself and demanded she touch him.

Another woman subsequently told a supervisor that the two officers ordered her into a Jetta while she walked her dog in Hollywood. They had arrested her in a previous encounter and she said she felt compelled to get in. Valenzuela allegedly got into the back seat with her, unzipped his trousers, forced her head into his lap and demanded oral sex, saying: "Why don't you cut out that tough girl crap."

In July 2012, police were tipped off by a member of the Echo Park neighbourhood watch that patrol officers were allegedly picking up prostitutes and releasing them in exchange for oral sex, the warrant said.

Investigators interviewed a third woman, who said Nichols had detained her in July 2011 and demanded oral sex, saying: "You don't want to go to jail today, do you?" Fearing arrest, she complied. She said Nichols had done the same thing to her six years earlier.

A fourth woman, a police informant, said she had sex with Valenzuela twice, once in her apartment, once in the back of an undercover police car. She said she feared going to jail if she refused. In fact she was sentenced to jail in April 2011, reportedly for cocaine possession, and lodged the lawsuit from jail.

The story broke just a week after the LAPD celebrated a drop in overall crime for the 10th consecutive year, which was seen by many as vindication of the force's rehabilitation since the 1990s.