In May, during BBC Radio 4's Today programme, the environmental activist and Guardian columnist George Monbiot challenged Myron Ebell, a climate sceptic at the Competitive Enterprise Institute, in Washington DC, to a £5,000 bet.

Gary Boisclair, an anti-abortion activist, and Gregg Iverson unsuccessfully challenged Ellison in the Democratic primary.

He and four other antiwar activists (including Dr. Benjamin Spock) challenged various provisions of the Selective Service Act.

Conservative activists challenged the authenticity of the documents used for the report.

OccupySTL marched in solidarity with the Veterans Day march earlier in the day but its activists challenged pervasive neglect of veterans.

In their absence, community groups and activists have repeatedly challenged the legitimacy of the election, on April 22, for mayor, City Council and a host of lesser offices.

However, artists and activists have challenged those conventional practices.

When activists challenged the system during the Khatami years, they found themselves isolated, a diminishing crowd without political support or mass mobilization to defend them.