Governors in several US states on Saturday called in National Guard troops as protests over police killings of black men intensified.

The protests began in Minneapolis following Monday’s death of George Floyd during an arrest.

Among those who called in the National Guard was Texas Governor Greg Abbott. The call came after several businesses in Dallas were looted and vandalized during a protest overnight Friday.

"Texans have every right to exercise their first amendment rights, but violence and looting will not be tolerated," the governor said in a statement.

Meanwhile, the mayor of Denver, Colorado, ordered a nighttime a curfew as demonstrations in the city entered a third day.

Mayor Michael Hancock said Saturday that the Colorado National Guard will help enforce the 8:00 p.m. to 5:00 a.m. curfew.

Downtown Denver has been the scene of clashes between protesters and police the past two nights. Some protesters broke windows and police fired tear gas, flash grenades and pepper pellets.

Other states to have called in the National Guard are Minnesota, Georgia, Ohio, Wisconsin, Kentucky and Utah.