The NUJ says it has uncovered more evidence of unacceptable police treatment of journalists covering protests.
It has called for an investigation into claims that police used force against the press during the Tamil protest outside parliament in London on Monday.
Legal officer Roy Mincoff said: “Yet again the police have ignored the guidelines agreed between ACPO and the media. We are now obtaining further reports from our members and will be taking action on their behalf.”
The parliamentary joint committee on human rights said in March it was deeply concerned that police seemed unaware of the guidelines about working with the media.
The home affairs select committee has also heard evidence from the NUJ and ACPO about the policing of the G20 protests in London last month.