Police brace to be 'guardians of democracy' as white supremacists descend on DC Police say they are hoping for the best, but preparing for the worst.

"Guardians of democracy."

That’s how the chief of the U.S. Park Police, Robert MacLean, described the role of law enforcement officers in the nation's capital this weekend as hundreds of white supremacists are expected to descend on the city to celebrate the one-year anniversary of last year's violent rally in Charlottesville, Virginia. Counter protests have also been planned across the city.

The "Unite the Right" rally in Charlottesville last year turned deadly when a 20-year-old Ohio man, James Alex Fields, allegedly accelerated his car into a crowd of counter-protesters, killing 32-year-old Heather Heyer and leaving 19 others injured, five critically. Fields has pleaded not guilty to federal hate crime charges and has also been charged under Virginia law with murder and other crimes. Fields is currently in jail awaiting trial.

Federal, state and local law enforcement have warned that violence could similarly mar this weekend’s events, but law enforcement agencies -- led by the Metropolitan Police Department -- are "going to do everything we can" to "make sure that everyone can participate in this event safely," MPD chief Peter Newsham told reporters on Thursday.

"We are intimately aware at MPD that the views being expressed on Sunday can strike at the core of a person’s values, and we are aware that these views can awaken a passion within," Newsham said, adding that people shouldn't let those personal passions overcome "civility."

Some law enforcement officials believe protesters speaking out against white supremacy could outnumber the white supremacists themselves, and extremists on both sides of the issue could disrupt otherwise peaceful rallies.

"Domestic terrorist actors and organizations have declared their intent to participate in the public gatherings," a warning issued by the Department of Homeland Security to law enforcement on Wednesday read. "Statements made by violent counter-protestors and [violent white supremacists] indicate they are seeking to initiate violent confrontations at the rallies to advance their ideological objectives."

The warning continued: "Law enforcement assesses the most vulnerable points and areas of highest likelihood for violence are likely to occur during the entering and exiting of rally sites and at meet up points and assembly areas being utilized by attendees."

According to Newsham, the key to preventing such violence is keeping the protesters and counter-protesters away from each other.

He noted that after the rally in Charlottesville last year, local police there "were criticized for failing to coordinate local and state resources, and failing to keep the two groups separate."

"Because we’ve had months to plan, and because this is something we do in Washington, D.C., on a regular basis, I can assure you that the planning and coordination amongst MPD and our local and federal partners has been seamless," he said.

Newsham said that in recent weeks, MPD officials have met with the city council, local religious leaders, business owners and others to "help alleviate the real and understandable concerns that they have."

"The law allows for First Amendment assemblies of any kind. The law does not allow for injuries to persons or destruction of property. And neither one of those behaviors will be tolerated," he added.

Newsham promised a "full deployment of officers and resources," saying his department is on "high alert" for anyone carrying weapons and "will continue to monitor all activities leading up to the event."

"We have a countermove for every move that any group could make to ... make sure that everyone can participate in this event safely," he said.

The director of the Homeland Security and Emergency Management Agency in Washington, Christopher Rodriguez, said his agency will be activating an emergency operations center on Sunday "for the duration of the event," and the center will be conveying up-to-the-minute intelligence to law enforcement on the ground.

The U.S. Secret Service and state and local authorities in Virginia are also assisting.

In its warning issued Wednesday, DHS urged law enforcement to look for "potential indicators of violent of activity," noting that "the majority of violent activity committed by anarchist extremists at public rallies involves the use of ‘black bloc’ tactics and methods, including dressing in black and masking the face to avoid law enforcement identification."

"Common tactics observed include assault, arson, the use of improvised incendiary devices such as Molotov cocktails, and the use of bricks, rocks, and other common items to assault law enforcement officers and damage law enforcement vehicles and property," the warning read.

In June, a federal grand jury in the Western District of Virginia charged Fields, now 21, with one count of a hate crime resulting in the death of Heather Heyer and 28 other hate crime-related counts for his alleged actions in Charlottesville.

According to the indictment, Fields drove from his home in Ohio to attend the "Unite the Right" rally at Emancipation Park in Charlottesville, which featured white supremacist leaders, and that he joined in "chants promoting or expressing white supremacist and other anti-Semitic and racist views."

The indictment states that prior to the attack, Fields obtained a series of social media accounts espousing the "belief that white people are superior to other races and peoples: expressing support of the social and racial policies of Adolf Hitler and Nazi-era Germany, including the Holocaust."

His social media accounts also "espoused violence against African Americans, Jewish people and members of other racial, ethnic and religious groups he perceived to be non-white," according to the indictment.

Attorney General Jeff Sessions issued a statement at the time of the indictment.

"At the Department of Justice, we remain resolute that hateful ideologies will not have the last word and that their adherents will not get away with violent crimes against those they target," Sessions said at the time.

ABC News' Josh Margolin contributed to this report.