New York City Mayor Bill de Blasio flew to Germany this week to participate in an anti-Trump protest, drawing criticism for leaving after a Bronx police officer was killed.

Democrat De Blasio will be the keynote speaker at a Saturday rally promoting social equality and climate protection, among others. The event coincides with a summit attended by President Trump and other world leaders in Hamburg. De Blasio's office announced his departure in a brief news release sent just hours before the mayor's Thursday evening flight.

The trip immediately raised questions from political opponents and from the city's police union, which has had a strained relationship with de Blasio.

"You've chosen to leave while the city is mourning. I just don't understand what he's thinking," Ed Mullins, president of the Sergeants Benevolent Association, told The Washington Post.

De Blasio left the day after Officer Miosotis Familia was fatally shot while sitting in a police vehicle in the Bronx early Wednesday morning.

Mullins questioned the purpose of the trip, saying de Blasio's job is in New York City, not abroad.

"It's not because he's playing an intricate role in solving world politics and world peace and all these issues. As mayor of New York, you really have no role in global politics," Mullins said. "Where you do have a role is in the city of New York and in the city of New York right now, a police officer is assassinated."

He added: "From a common sense standpoint, Why are you not in Germany? Why am I not in Germany? Because we don't have business in Germany. The question is what is the business of the mayor in Germany? To discuss what?"

De Blasio also skipped the swearing-in ceremony of 524 new police recruits, which de Blasio's political opponent, Republican state lawmaker Nicole Malliotakis, immediately slammed.

Malliotakis, who's running to be New York City's mayor, went further by sharing a Photoshopped picture showing de Blasio holding a plate of sausages.

"While #NYC's subways crumble, sex crimes increase double digits, litter on streets pile up & the number of homeless soars . . . #G-20," she wrote.

The New York Post delivered a stern message on its front page Friday: ". . .DON'T COME BACK!"

Donald Trump Jr. also weighed in:

"NYC mayor goes to G20 to protest against capitalism while the City's education, infrastructure etc rots under his "progressive" stewardship!"

De Blasio's press office did not immediately respond to a request for comment, but spokesman Eric Phillips said on Twitter that de Blasio made the decision as soon as he was sure the trip would not prevent him from attending Familia's funeral Tuesday. De Blasio returns Sunday.

De Blasio also addressed criticisms at his regular radio appearance on WNYC on Friday.

"All the issues that need to be attended to I'm attending to every day regardless of where I am," de Blasio said, speaking from Hamburg's city hall.

De Blasio told WNYC that local and state officials in Hamburg invited him about 10 or 12 days ago to talk about issues such as climate change and immigration.

"They invited me as a colleague. They wanted to, I think, represent the fact that there are a variety of views in the United States . . . particularly on climate change," de Blasio said, adding that his role is to impart the point of view that issues such as climate change have to be addressed locally. De Blasio and other mayors from across the country have vowed to uphold the Paris agreement, sidestepping Trump after he withdrew the United States from the climate deal.

Organizers of the rally where de Blasio will be speaking said the mayor was invited because of his political views that contradict Trump's nationalist policies.

"Bill de Blasio is the mayor of a city that stands like no other on our planet for liberty, internationality and the coexistence of different cultures - with all the challenges involved," the group called Hamburg Shows Attitude said on its website. "For example, de Blasio is committed to social equality, minority rights and climate protection. New York is a world symbol of freedom and diversity. Right now, it is important to stand together, regardless of nationality, for values such as cohesion and nonviolence, where they are under such intense pressure around the world."

Trump has been the target of many of the protests in Hamburg, where German Chancellor Angela Merkel is hosting Trump and other world leaders for the Group of 20 summit.

German security forces in riot gear clashed with thousands of anti-capitalist protesters Thursday, using water cannons and pepper spray to clear a march in which a militant group with anarchist sympathies had a prominent presence, The Post reported. Riots continued when the summit began Friday as protesters burned cars and blocked roads. First lady Melania Trump was forced to miss an event with the spouses of other world leaders because protesters blocked her from exiting the guesthouse where she and the president are staying.

De Blasio said he meant to announce the trip earlier, but Familia's killing changed that plan.

"It was important to focus on that and deal with that painful reality," de Blasio said. "I didn't want to do anything until I know when the services will be."

Still, the trip seems to further strain an already combative relationship between de Blasio and the city's police force.

Police union officials have accused de Blasio of fueling anti-police sentiment, going back to his 2013 campaign, in which he heavily criticized the police department's "stop and frisk" tactic.

The tension boiled over in December 2014 after two Brooklyn police officers were killed while sitting in their squad car.

"There's blood on many hands tonight," Patrick Lynch, president of the Patrolmen's Benevolent Association, said the night of the officers' deaths. "Those that incited violence on the street in the guise of protest, that tried to tear down what New York City police officers did every day. We tried to warn it must not go on, it shouldn't be tolerated. That blood on the hands starts at the steps of City Hall in the office of the mayor."

Later that month, hundreds of police officers outside Christ Tabernacle Church in Queens turned their backs as de Blasio spoke at the funeral of Rafael Ramos, one of the slain officers.

The rift has yet to show significant signs of closing even as the mayor's office announced a $1.3 million investment to install bullet-resistant door panels and other layers of protection to all NYPD vehicles. The mayor's office announced the funding the same night he left for Germany.

Mullins, of the police union, said officials have been slow to add safety measures to the city's 4,000 police vehicles. A little more than 2,000 vehicles have been outfitted with bullet-resistant door panels, and the city plans to have the protections on every patrol vehicle by the end of the year, according to the mayor's office.

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Mark Berman, Christian Caryl, Isaac Stanley-Becker and Karen Tumulty contributed to this report.