In our not-so-distant history, protest in the United States was handled by local law enforcement that treated demonstrations and marches as mere nuisance, mediating and directing as needed. Today, observing the interaction between Occupy movements and law enforcement suggests something different is afoot. Present Occupy protests are now being defined by a bewildering set of law enforcement strategies – and current practices display a worrying new trend.

While riot police are not necessarily an everyday feature at any given protest, the sheer frequency with which we are witnessing their presence on city streets throughout the United States is enough to give average citizens cause for concern; the excessive force being routinely deployed is alarming.

Within the first few days of Occupy Wall Street, protesters began to notice the presence of the NYPD's Counter Terrorism Unit at Liberty Plaza. Joanne Stocker, who has become a fixture since day one at Wall Street, recalls within the first few days waking up to a Counter Terrorism Unit van, parked on the fringes of Liberty Plaza, which was taking video of her and her friends while they slept.

Protesters at other Occupy encampments give similar accounts. Robin Jacks, a member of Occupy Boston's media team, relates being photographed multiple times by police. Dustin Slaughter, who has spent time both at Occupy Wall Street and Occupy Philadelphia, attests to the presence of the NYPD Counter Terrorism Unit at Liberty Plaza, saying that the Counter Terrorism Unit have been at Liberty Plaza filming on a regular basis. Slaughter also comments: "Philadelphia Police Homeland Security Units have had a regular presence at the Occupy Philadelphia encampment."

Protesters are indeed correct to view the law enforcement they encounter at Occupy with a critical eye. The USA Patriot Act, which had its 10-year anniversary last week, gave the US government virtually unchecked powers to spy and track the activity of ordinary Americans without probable cause right after the 9/11 attacks. For that reason, it should come as no surprise that law enforcement agencies – thus empowered – have shown up at various Occupy protests armed with cameras, most certainly, to keep surveillance on protesters who are merely exercising their first amendment rights.

Reports of targeted arrests of informal "leaders" at Wall Street, Chicago and Boston indicate surveillance measures are operating. In Boston and Chicago, reports of extended and humiliating detentions of targeted occupy "leaders", typically from Direct Action, media, legal and medics groups, are disturbing. Dan Massoglia of the Occupy Chicago media team further reports that arrested individuals were deprived of their phone call, food and water, and that mattresses were removed from cells, while one woman was placed in solitary confinement.

Curfews placed on occupied city parks are equally perplexing. Legislative Plaza, the site for Occupy Nashville, was ordered to be shut down between 10pm and 6am, rendering its occupation impossible. The orders, however, did not follow standard procedure. Instead of being issued by Nashville municipality, the order came from the state of Tennessee Department of Safety and Homeland Security.

Nancy Murray, director of education at the Massachusetts chapter of the ACLU, views the various signs of Department of Homeland Security involvement as important indicators that the federal government is orchestrating the policing of Occupy protests throughout the country.

"This would be a big concern because it would show that the federal government is possibly playing an active role in opposing people's rights to free speech and to peaceably assemble," says Murray.

Does this mean that protesters are being treated as terrorists? "It's too early to tell," says Murray. "But it's obvious the feds are watching and observing to get more information … It is possible that the Joint Terrorism Task Force is calling the shots."

"At the beginning of this movement, I could understand why there might have been a presence of Counter Terrorism Units operating at Liberty Plaza – because nobody knew who we were and what we represented," states Stocker. "Now, their presence is just overkill and antagonistic. What we stand for is clear and it is clear we are not terrorists."

Occupy protesters should make themselves familiar with the USA Patriot Act. Section 802 expanded the definition of domestic terrorism to include persons who engage in acts of civil disobedience to coerce or affect the conduct of government by intimidation of the civilian population. Furthermore, the US Department of Defence training manuals, until an amendment in 2009, equated protest with "low-level terrorism". Although the DoD changed the wording two years ago, human rights lawyers and activists have lingering concerns about whether the sentiment and intent has caught up with the change.

Finally, there is the disquieting issue of excessive force at Occupy. In the autumn of 2008, the Army Times reported that for the first time, the US Army planned to station an active unit under the control of Northern Command serving as an on-call federal response in times of both natural and man-made emergencies, including terrorist attacks. Training included a non-lethal package, elements of which the US Army has been using in Iraq, designed to subdue unruly individuals. "The package includes equipment to stand up a hasty road block; spike strips for slowing, stopping or controlling traffic; shields and batons; and, beanbag bullets."

Despite the existence of the National Guard, whose raison d'etre is to augment civilian law enforcement when its capabilities are exceeded, this additional unit, according to the Army Times, may be called upon to help with civil unrest and crowd control. The excessive force exhibited at some Occupy locations – the use of tear gas; alleged use of rubber bullets and reported presence of sonic weapons – is becoming a pattern. A protester in California, who wished to remain anonymous, recalls experience of a long range acoustic device (LRAD) in Oakland last week:

"I had been tear gassed three times, so when I first saw the sound cannon, I panicked. When the cannon went off, I felt it pulse through me and I instantly felt dizzy and nauseated. At one point, I fell over. I noticed others around me had fallen over as well and some vomited."

Such anti-riot technologies were characterised as inhumane by human rights observers when they were used to subdue unarmed, peaceful protesters during civil unrest in Tbilisi, Georgia, in 2007. They have no place in a democratic country. They may be characterised by authorities as "non-lethal", but they can all too easily become lethal if misused by reckless law enforcement agents. The Occupy movement is explicitly a nonviolent exercise of first amendment rights, yet its policing bears all the hallmarks of a chilling militarisation of law enforcement in the United States.