Protestors hold banners outside the Prudential Building which houses the reelection headquarters for President Barack Obama during a demonstration against the policies of NATO Summit in Chicago, the United States, on May 21, 2012. [Xinhua]

Protestors march to the Prudential building which houses the reelection headquarters for President Barack Obama during a demonstration against the policies of NATO Summit in Chicago, the United States, on May 21, 2012. [Xinhua]

A protestor holds a national flag upside down outside the Prudential Building which houses the reelection headquarters for President Barack Obama during a demonstration against the policies of NATO Summit in Chicago, the United States, on May 21, 2012. [Xinhua]