Chief Constable of Greater Manchester Police calls for more resources.
Ian Hopkins says his officers are under "real strain".
Police in Greater Manchester are under "real strain'' after the loss of nearly one in four officers in recent years, the force's chief constable has said.
Ian Hopkins cited official figures which show that GMP - currently carrying out one of its biggest ever investigations - had 8,148 officers in March 2010, but this had dropped by 23% to 6,297 by March last year.
Writing on Twitter, he praised his officers for their "outstanding'' work policing the Parklife festival over the weekend along with a protest against Islamist terrorism.
He wrote: "Outstanding from @gmpolice officers & staff this w/e policing protest & Parklife. Real strain on everyone not just this weekend.''
Under his tweet he posted a page from the HM Inspectorate of Constabulary (HMIC) 2016 efficiency report for GMP, with a table setting out the staffing statistics.
It shows that GMP's total workforce fell by a fifth during the period, from 13,189 to 10,506, while the number of community support officers (PCSOs) reduced from 842 to 748 - a drop of 11%.
The table shows these are set to fall even further over the next three years, with the total workforce expected to be 10,108 by March 2020 with 500 PCSOs.
Manchester mayor Andy Burnham tweeted: "@gmpolice are stretched to limit & in middle of on-going investigation.''
He criticised the protest against Islamist terrorism, organised by former English Defence League (EDL) leader Tommy Robinson, which drew a counter-demonstration.
"These EDL-types who came today need to have a look at themselves,'' Mr Burnham added. "@gmpolice deserve better.
"I care about our Police being unnecessarily distracted when they are worn out & still working hard to investigate a major incident.''
The force, the second largest in England, is investigating the Manchester Arena suicide bombing which left 22 people dead and more than 200 injured.
The figures show that GMP will lose more staff than the national average, with the total workforce forecast to drop by 24% between 2010 and 2020, compared with 21% across England and Wales.
About 400 officers were deployed to police Sunday's protest and officers made eight arrests for public order offences.
Chief Superintendent John O'Hare said many officers who had already worked long hours had to operate in "extremely challenging circumstances''.