LIMA (Reuters) - Peru's top police officer resigned late on Wednesday as President Ollanta Humala faced increasing pressure over rising crime in the Andean nation after two high-profile murders.

The killings further eroded confidence in the police force and hurt Humala's approval rating, which fell to a three-month low of 51 percent over concerns he is not doing enough to fight crime, according to a Datum poll published this week.

Opposition lawmakers and even some of Humala's centrist allies had called for the resignation of police chief Raul Salazar and Interior Minister Wilfredo Pedraza. Pedraza has not quit despite calls for his removal from some congressmen.

"Police reforms are on the way and they should be strengthened and supported by everyone," Salazar wrote in his resignation letter, a copy of which was circulated by the Interior Ministry.

This week's Datum opinion poll was conducted several days after a prominent photojournalist was gunned down in front of his home. The murder came days after a businessman was shot during a robbery yards from a police station.

It is still unclear why the photographer, who worked for leading newspaper El Comercio, was killed.

Humala, a former military officer, is one of the most popular presidents in Peru in years.

His approval rating has dipped several times on security issues including violent conflicts over mining projects in the highlands and the government's bungled attempts to stamp out a leftist insurgency that oversees drug trafficking.

Humala has raised police officers' salaries and boosted training and patrols, but corruption and crime remain widespread.

Crime in metropolitan Lima doubled between 2000 and 2011, with kidnappings and homicides about tripling, according to government data.

According to the Datum poll, 45 percent of Peruvians think the government is doing nothing to stop crime, up from 36 percent in November 2011, soon after Humala took office.

The survey of 1,304 people was taken between February 27 to March 1 and had a margin of error of 2.7 percentage points.

(Reporting by Marco Aquino; Writing by Mitra Taj; Editing by Doina Chiacu)