"Tonight, it appears that two snipers shot 10 police officers from elevated positions during the protest/rally. Four officers are deceased, two are in surgery and three are in critical condition," Dallas Police Chief David Brown said in a statement.

"An intensive search for suspects is currently underway. No suspects are in custody at this time. Please keep us in your thoughts and prayers tonight," he added.

Ten police officers were shot in Dallas and arrests were made in New York late light day, media and officials said, as thousands gathered in major US cities to demonstrate against the second fatal police shooting of a black man in two days.

Most of the protests were largely peaceful but tension was evident after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late on Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.

In Dallas, 10 police officers were shot during demonstrations at Belo Garden Park, television station KDFW TV reported. The condition of the officers was not known, it said. Footage showed a heavy police presence with officers taking cover behind vehicles on the street.

Soon after, CNN reported that three Dallas transit officers had also been shot during the demonstration. Their condition was not known, it said.

In Chicago, protesters shut down a stretch of the Dan Ryan Expressway - one of Chicago's main arteries - for about 10 minutes on Thursday.

In New York, several hundred protesters blocked traffic in Times Square in the heart of Manhattan, chanting "Hands up, don't shoot." More than a dozen arrests were made, the New York Police Department said.

In St. Paul, about a thousand people gathered outside the governor's mansion, chanting "Hey hey, ho ho, those killer cops have got to go," and other slogans.

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