"Our city is grieving tonight," St. Louis Mayor Cara Spencer, who was sworn in a month ago, told reporters. "The loss of life and the destruction is truly, truly horrendous," Reuters reported.
The number of injured was undetermined as officials said they were still gathering information from several hospitals where people were taken.
The city imposed a curfew from 9 p.m. to 6 a.m. (0200-1100 GMT) in two police districts where most of the damage took place to prevent injuries from dangerous debris and reduce the potential for looting.
About 500 first responders, including 17 search teams, were deployed, and early indications suggested the number of injured was relatively small considering the overall damage, Fire Chief Dennis Jenkerson said.
The National Weather Service reported at least half a dozen tornadoes touching down in Missouri and neighboring Illinois and other severe weather stretching all the way to the Atlantic Coast, including another tornado in New Jersey.