CHICAGO –  Two more individuals have been arrested and accused of threatening the NATO conference in Chicago, in the latest escalation of tension between police and protesters on the sidelines of the Afghanistan war summit. 

As President Obama arrived at summit headquarters Sunday to meet with Afghan President Hamid Karzai as well as top NATO officials on the way forward in Afghanistan, protesters were kicking off what is billed as the largest demonstration of the weekend. The latest arrests and heavy police presence were used as a rallying cry, as lead protester Andy Thayer called on Obama to call off the cops. 

"We are holding you, President Obama and Mayor (Rahm) Emanuel personally responsible for any violence," he said. "If you value the election this November, you'll tell your officers to stand down." 

A slew of demonstrators and plotters have already been brought into custody, some on serious charges. 

Prosecutors previously charged three men with planning to attack President Obama's campaign headquarters, Mayor Rahm Emanuel's home and other targets. They're accused of trying to make Molotov cocktails. 

Two more alleged plotters, Sebastian Senakiewicz and Mark Neiweem, have also been charged. The Cook County State Attorney's office said in a statement Sunday that Neiweem, 28, is charged with attempted possession of explosives or incendiary devices and Sebastian Senakiewicz, 24, is charged with falsely making a terrorist threat. It was unclear if the latest case was related to that of the other three men. 

Increasingly tense clashes Saturday night tested police who used bicycles to barricade off streets and horseback officers to coax them in different directions. Eighteen people were arrested, Police Supt. Garry McCarthy said. 

Organizers of Sunday's rally had initially predicted tens of thousands of protesters this weekend. 

But that was when the G-8 summit also was scheduled to be in Chicago. Earlier this year, Obama moved the Group of 8 economic meeting to Camp David, the secluded retreat in rural Maryland. 

Chicago kept the NATO summit, which will focus on the war in Afghanistan and other international security matters, but not the economy. That left activists with the challenge of persuading groups as diverse as teachers, nurses and union laborers to show up for the Chicago protests even though the summit's main focus doesn't align with their most heart-felt issues. 

"I'm here to protest NATO, which I feel is the enforcement arm of the ruling 1 percent -- of the capitalist 1 percent," said protester John Schraufnagel, 53, who took a bus from Minneapolis to Chicago and was among the first demonstrators to gather at Grant Park Sunday. 

Sunday's protest followed several, smaller demonstrations the previous two days including one peaceful march to the home of Emanuel, Obama's former chief of staff, on Saturday. But a march later that evening involving hundreds of demonstrators stretched for hours as protesters zigzagged back and forth through downtown, some decrying terrorism-related charges leveled against three young men earlier in the day. 

McCarthy said police would be ready with quick but targeted arrests of any demonstrators who turn violent Sunday. 

"If anything else happens, the plan is to go in and get the people who create the violent acts, take them out of the crowd and arrest them," warned McCarthy. "We're not going to charge the crowd wholesale -- that's the bottom line." 

Security has been tight throughout the city. As police gathered en masse on street corners, near parks and key landmarks, the city's streets remained largely vacant and many downtown buildings closed. 

The Associated Press contributed to this report.

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