Is the government ever justified in restricting freedom of speech or of the press?

Freedom of speech is often considered to be one of the most basic tenets of democracy. As a fundamental right it is enshrined in documents such as the Bill of Rights in the United States, the United Nations Declaration of Human Rights, and the European Convention on Human Rights ... Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances. (1st Amendment to the US constitution - 15 December 1791)