Unison said the 24-hour drinking culture had affected A&E departments
Hospitals in some of Britain's biggest cities are spending tens of thousands of pounds on police cover at accident and emergency departments at weekends.
NHS organisations in Glasgow, Newcastle and Merseyside revealed they paid for A&E cover on Friday and Saturday nights in a Freedom of Information request.
Officers are employed to stop violence towards doctors, nurses and other staff, the NHS trusts and boards said.
A Unison spokeswoman said staff said A&E was "like a war zone" at weekends.
The 24-hour drinking culture had meant more people were coming to A&E drunk and it could be "very frightening" for staff and other patients, she said.
"It's a sad fact that A&E need to have police officers in there to protect staff and patients."
The union, which represents 450,000 health workers in the UK, wants people to be prosecuted and given heavier sentences if they are convicted of assaults on NHS staff and paramedics, she added.
A spokeswoman for NHS Greater Glasgow and Clyde said it had introduced a police presence in two A&E departments in Glasgow because of a "significant rise in the number of staff reporting physical or verbal abuse".
The board, which spends £60,000 a year on police officers, set up "the most comprehensive violence and aggression policy in Scotland" in 2005 - which included CCTV and freephone lines direct to local police stations - she said.
She continued: "Through our violence and aggression policy we offer staff the opportunity to complete a City and Guilds Certificate in dealing with aggression.
"As a last resort, staff are able to withhold treatment from a patient who is persistently violent or aggressive."
Newcastle upon Tyne Hospitals NHS Foundation Trust pays £25,000 a year for a police officer for Thursday, Friday and Saturday nights at Newcastle General Hospital.
Paul Brewis, operational services manager at the trust, said: "Due to the location of the A&E department this initiative was introduced to enhance existing security arrangements on site, to support frontline staff who may be at risk from a number of attendees and potentially subject to abuse. verbal or physical, as well as providing additional site surveillance."
A spokesman for the Royal Liverpool and Broadgreen University Hospitals NHS Trust - which spent £22,950 on police officers in 2009 - said the additional support had been in place since the late 1990s.
One police officer covers the Royal Liverpool University Hospital on Friday and Saturday nights, between 2200 and 0600, he said.
Other trusts said they paid for police to be on site during the week.
Dr Peter Harrowing, from the University Hospitals Bristol NHS Foundation Trust's Legal Services, said the trust paid £50,000 for two police liaison officers from Monday to Friday between 0800 and 1700.
Avon and Somerset Police subsidised the remainder, he said.
Sue Frith, from the NHS Security Management Service, said the Service provided support to the NHS on how to tackle violence through a network of professionally trained specialists.
"There is no such thing as a general security level in NHS A&E departments.
"Each part of the country will encounter different needs at different times of the year, from the flu pandemic in winter months to a seaside town in the height of summer. Trusts are able to identify the risk and staff accordingly," she said.