Nixon said yesterday that though many protesters were making themselves heard peacefully, the state would not allow looters to endanger the community where 18-year-old Michael Brown was shot in a street.
The curfew will run from midnight to 5 am (local time) today.
"I am committed to making sure the forces of peace and justice prevail," Nixon said at a chaotic press conference at a church that was interrupted repeatedly by people objecting to the curfew and demanding that the officer who shot Brown be charged with murder.
Nixon's curfew announcement came after tensions again flared in Ferguson late Friday night. Local police had earlier in the day identified the officer who shot Brown as Darren Wilson and released documents and video footage alleging that Brown had robbed a convenience store just before he was shot.
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Police said Wilson was unaware Brown was a suspect when he initially encountered him walking in the street with a friend. Nixon said the US Department of Justice is beefing up its civil rights investigation of the shooting.
Johnson assured those at the news conference that police would not enforce the curfew with armored trucks and tear gas but would communicate with protesters and give them ample opportunity to leave.
Nixon and Johnson were flanked by numerous local elected officials, including US Rep William Lacy Clay Jr, who urged Johnson to be flexible with the midnight curfew.
Among the many people shouting questions was Malik Shabazz, the president of Black Lawyers for Justice, who said that members of his group and the New Black Panther Party and the Nation of Islam had been helping to maintain order and deter protesters from violence.