FERGUSON, Mo. — Gov. Jay Nixon of Missouri on Saturday imposed a midnight to 5 a.m. curfew in this small city, declaring a state of emergency as violence flared anew after a week of street protests over the killing of an unarmed black teenager by a white police officer.

“This is not to silence the people of Ferguson, but to address those who are drowning out the voice of the people with their actions,” Mr. Nixon, a Democrat, told reporters and residents at a Ferguson church. “We will not allow a handful of looters to endanger the rest of this community. If we’re going to achieve justice, we must first have and maintain peace.”

Mr. Nixon added: “This is a test. The eyes of the world are watching.”

As the curfew took effect, heavy rains began to fall and the streets, which had been filled with people less than an hour before, were largely deserted.

Capt. Ronald S. Johnson, the state Highway Patrol commander whose officers have overseen public security in Ferguson since Thursday, did not say how long the curfew would be in force or whether violators would be arrested.