Security that surrounds the Donald Trump inauguration is the most challenging of any inauguration in recent history, said senior officials involved in the planning. This is in large part due to the same things that took place during the presidential campaign.

Beside the daunting tasks of preparing for threats of an inaugural ceremony, the agencies that are responsible for the January 20 security, that number more than three dozen, are preparing for large numbers of protesters arriving at the U.S. capital as well as what might be close to one million Trump supporters.

The agencies are concerned over the possibility of the two groups having confrontations due to them being deeply divided from the election and during a time when millions across the globe will be focused on Washington.

At the least, officials have said that protests would add more pressure on the area’s already stretched security network.

A former homeland security secretary said that each inauguration has it risks and this one is no exception.

However, the past few inaugurations have not faced the large number of protesters expected to take part in this on January 20.

For example, the crowd at President Barrack Obama’s 2009 inauguration was close to 2 million people but few were protesters and no one was arrested.

The National Park Service controls much of Washington, D.C. public land from the National Mall to the sidewalks and already has seen 23 requests for permits from groups that hope to host events both in support of and against Trump.

During other inaugurations, there are only a handful of different permit requests.

Safeguarding a peaceful transfer of the nation’s power is not easy even under predictable circumstances. A new president’s first inauguration is likely one of the highest profile rituals in U.S. public life.

There is expected to be tens of thousands of law enforcement personnel from Washington’s Metropolitan Police to the FBI and the National Park Service working to protect Trump’s inauguration and the many related activities.

In all, over three dozen different agencies will be spread across the capital working to prevent the big occasion from becoming the platform for one person or groups of people wanting to cause harm.

Work began months ago and has taken a new urgency since Trump’s election win and soon will include a perimeter of security around the Mall, the Capitol and a large part of the entire city.

Cost of the security along is expected to surpass $100 million.