After events in Charlottesville, Va., last weekend showed how much violence white nationalist rallies could provoke, police chiefs from Richmond, Va., to Boston were taking steps to avoid a repeat of a situation in which the police appeared to have little control of the crowd.

Texas A&M University canceled a “white lives matter” rally at which Richard Spencer, a white supremacist leader, was to appear, citing safety concerns. Officials in Mountain View, Calif., where Google has its headquarters, were gearing up for one of several marches at the company’s offices around the country to protest the firing of a male employee who wrote a memo criticized as sexist.

Rallies like the one in Charlottesville, fueled by overt displays of racism, attended by members of self-described militias, and attracting counterprotesters, pose novel challenges: Many of the demonstrators are legally and openly carrying firearms, including semiautomatic weapons. And instead of protesters versus police, as has often been the case in recent years, the situation is civilian versus civilian, with some participants spoiling for a fight.

But to deal with these new circumstances, the police have few new tactics.

Crowd-control techniques are much the same, experts said, whether demonstrators are armed or not. A crucial technique is keeping opposing sides apart, which the police tried and failed to do in Charlottesville on Saturday. In the hours leading up to the planned rally, people fought in full view of police officers. On Monday, a man was charged with driving a car into a crowd of counterdemonstrators, killing a woman and injuring more than a dozen others. The actual rally was called off by the police after the governor declared a state of emergency.