The American Civil Liberties Union (ACLU) has released documents they say shows the FBI has monitored the activities of various domestic activist groups.

The ACLU says it has obtained more than 2,300 pages of files as part of a lawsuit filed under the federal Freedom of Information Act.

The heavily-edited documents show the FBI has investigated organizations such as People for the Ethical Treatment of Animals and the environmental group Greenpeace. Some of the groups placed under surveillance had planned demonstrations at last year's Democratic and Republican presidential conventions.

The FBI says it monitored the groups to determine if they had any links to any extremist groups that engaged in violent or illegal acts.