PITTSBURGH - As the Group of 20 summit of the world's leading economies went into its second and last day, police and anti-G20 protesters took to the tweets.

Both have harnessed popular microblogging service Twitter, the police using it to track the movements of hardcore anti-G20 protesters and the demonstrators to rally support for their anti-capitalist cause.

The protesters tweeted all day Thursday and into the small hours of Friday, piecing together a snapshot of an unfolding siege in the streets and parks of Pittsburgh as law enforcement cracked down on young anti-G20 activists.

"Two more arrests, cops have blocked off student dorms, using weapons," read a tweet posted on the G20pgh feed, set up by the Pittsburgh G20 Resistance Project, as police broke up a demonstration near the University of Pittsburgh.

The tweets told protesters when a new shift of police was coming on duty. They shouted out messages like: "RIOT POLICE MARCH DOWN FORBES, SENDING STUDENTS SPRINTING TOWARD TOWERS. TEAR GAS SHOT AT CROWD."

"They've been putting out, making up all sorts of addresses and pieces of information, but any open source of information out there is always a good tool," said Pittsburgh police Sergeant Lavonnie Bickerstaff.

"Police scanner is onto group forming on Hot Metal Bridge," read another tweet from the protesters.

An hour later it was followed by a mocking message, in capital letters, that read: "SCANNER JUST SAID: BE ADVISED WE'RE BEING MONITORED BY ANARCHISTS THROUGH SCANNER."

Meanwhile, as hundreds of riot police held back the protesters on the streets, others back at the Joint Information Center were making like the protesters: they, too, were tweeting.

"We're using Twitter as a monitoring tool, not to have two-way conversations with the protesters," said Lieutenant Sue Kerver of the US Coast Guard, one of several branches of the security forces which are in Pittsburgh to beef up security for the G20 summit.

"By us monitoring them and seeing what's happening at street level, we can correlate that information with what we have coming in from our folks," Kerver said.

During the daytime protests, a police helicopter hovered high in the air above parts of neighborhoods in northern Pittsburgh, where a hardcore group of demonstrators were still taking on the riot squads, hours after most of the marchers had been dispersed when officers fired off pepper spray and non-lethal rounds.

The G20 summit marked the first time the Pittsburgh police has used Twitter "as a collaborative effort with other agencies," said Bickerstaff, although the bureau uses it regularly as part of routine intelligence gathering.

But Bickerstaff refused to say how much of a help monitoring the protesters' tweets has been.

"It is part of intelligence. If we tell that, it wouldn't be intelligence anymore," she said.

Sixty-six people have been arrested following the anti-G20 protests, the police said earlier -- in an emailed statement.

A tweet from the protesters called for supporters to head down to the local jail for a vigil for the arrested activists.

"Folks have been there & up all night. First Ave & Municipal Courts Dr. Please come!" the tweet begged.