At least 50,000 patients have had medical appointments cancelled across the Republic of Ireland on Tuesday due to a 24-hour nurses' strike.
It is the second in a series of strikes in a dispute over pay and staff shortages.
The Irish Nurses and Midwives' Organisation (Inmo) has extended the strike to include respite centres for the elderly and the disabled.
About 37,000 Inmo members are taking part in pickets at 240 locations.
All outpatient, inpatient and day surgery appointments have been cancelled and injury units are closed.
Emergency departments are open, but with reduced nursing staff cover.
The Health Service Executive has asked that people only attend if absolutely essential.
More than 80 nurses were on the picket line at Letterkenny University Hospital in County Donegal on Tuesday.
One nurse, who works in the Emergency Department, said understaffing in the hospital was putting patients at risk.
"On some days there is one nurse to 30 patients, that's not acceptable," she said. "We need to be able to provide safe care for all patients."
Ciaran McLaughlin, from the Inmo striking committee at the hospital, said striking nurses had been overwhelmed by the support of the public.
"Nurses are working in the hospital looking after their families and relatives," he said.
"We're not able to provide the care we want to provide because we are understaffed and staff are burned out."
Thousands more patients will be affected if the next six days of planned industrial action go ahead.
The Psychiatric Nurses Association (PNA) will also be staging an overtime ban in mental health facilities over the next three days.
On Monday, the government offered to go back to talks on workplace issues relevant to nurses - but categorically ruled out any discussions on pay.
However, both the Inmo and the PNA dismissed the overture as cynical, saying that talks without pay would not resolve the recruitment and retention crisis.