"I can't tell you how (the directive) happened because clearly I'm campaigning at this point in time," Prentice told reporters Tuesday at an Edmonton party rally for the May 5 election.
"(But) everyone in this province has the right on their own time to be involved as a volunteer in political service with a party of their choice.
"No employer, including the government of Alberta, has the right to intervene and demand answers. I consider it ridiculous and I won't stand for it."
A CBC report quoted an Alberta Public Service Commission spokesperson as saying the directive was to protect staff from accusations of inappropriate conduct or from straying into questionable activity.
Brian Jean, leader of the Wildrose party, said the directive reflects a government mindset that considers public servants to be pawns for partisan political ends.
"After 44 years of the same government, what they think is that the bureaucracy belongs to the PC party of Alberta," Jean said at a news conference at the legislature.
"It doesn't. The public service belongs to the public."
Jean said Prentice has shown similar disrespect to the democratic process by ordering an independent legislature committee in February to reverse a decision on funding for the auditor general.
The Progressive Conservatives have also failed to pass laws and pointed to 66 bills, including the 2012 Education Act, waiting to be proclaimed, he added.
Jean also accused the PCs of subverting their own public disclosure rules by quietly creating a new spending category that details food and beverage costs for employees during business meetings. The category is exempt from public disclosure.
Jean, handing out copies of government documents obtained through freedom of information, said the Tories have used the category to hide $6 million in expenses over the last two years.
“The gold standard of disclosure was actually the gold standard of hiding,” he said.
Prentice said the category was created in 2012 before he joined government.
"It (the category) should be eliminated," he said. "I'm not sure where it came from. It predated me as premier and I don't think we need it."
Prentice, asked about the lag in proclaiming bills into laws, noted it takes time to draft regulations that support legislation.
Two other leaders also campaigned in the Alberta capital Tuesday.
The NDP's Rachel Notley said her party would add 2,000 long-term care spaces over four years to reduce the wait list and relieve pressure on overcrowded hospitals. She said hundreds of patients are being forced to spend months in hospital waiting for room in a nursing home because the PC government has ignored the issue for too long.
Liberal Leader David Swann was campaigning and shaking hands in the city's west end.