Liberal MP Frank Valeriote says it was an "oversight" that his party did not identify that it was behind an anti-Conservative robocall used during the last federal election Guelph, Ont.

The riding is already the focus of an Elections Canada probe into opposition accusations that the Tories used automated messages to direct voters to the wrong polling stations. The Conservatives deny the accusations.

Now the Liberals are left to explain the automated message, or robocall, that was critical of Conservative candidate Marty Burke.

Audio of the recording surfaced Friday night.

The recording does not state that it came from the Liberal Party or from Valeriote's campaign. The Elections Act requires parties or candidates to identify themselves in campaign ads.

In the recorded message from last spring, a female voice warns against Burke's anti-abortion views.

The woman identifies herself as Laurie, or Lori, MacDonald and says she's been an advocate for women's rights nearly all her life.

"Conservative candidate Marty Burke believes that absolutely under no circumstance, should a woman have the right to choose," the message says. "Government should not be able to force their beliefs onto me."

The message goes on to say that the race in Guelph is close.

"Vote strategically on Monday to protect our hard-earned rights from the Conservatives and Marty's extreme views."

Valeriote won the riding.

On Saturday, Valeriote told CTV News that it was an "oversight" that the party and candidate were not identified on the call.

The Liberal message is the latest development in the ongoing robocall scandal. Elections Canada has launched an investigation into suspicious calls voters received during the election after receiving a high volume of complaints.

The investigation initially centred on the Guelph, Ont. riding, where a number of voters reported getting automated calls that told them to go to non-existent or incorrect polling stations on election day.

Since then, Canadians in dozens of ridings have reported receiving similar calls, or being telephoned late at night and harassed by people purportedly calling on behalf of political parties.

Elections Canada recently confirmed it has received more than 31,000 contacts after MPs and political parties encouraged Canadians to report any misleading or harassing calls they may have received during the election.

Opposition parties and the Conservative government have blamed one another for the calls, trading accusations of "dirty tricks" and voter suppression tactics.

The website Leadnow.ca is encouraging Canadians to write to Prime Minister Stephen Harper and opposition party leaders and demand a full investigation.

On Saturday, Conservative MP Dean Del Mastro called on the Liberals to explain the message.

"This is a dishonest, sleazy robocall that was made," Del Mastro told CTV. "It was an attack piece that disguised who in fact made the call."

Valeriote denied hiding the source of the automated message.

"This was issue based, not voter-suppression based," he told CTV. "It's two different things, and everyone has acknowledged that."

Protests planned

Protests will be held in cities across Canada on Sunday in response to the growing scandal.

Rallies have been planned in at least 27 cities, including Halifax, Toronto, Vancouver and Calgary, to demand a full inquiry into the allegations that misleading calls were placed to voters.

A small group of protesters got a head start in Lethbridge, Alta. on Saturday, rallying in front of Conservative MP Jim Hillyer's constituency office with signs and a mock casket carrying the "corpse of democracy."

The protesters said they weren't targeting a specific political party, but want to send a strong message to the government about robocalls.

"Whoever is doing it, whatever party they may be connected to or not connected to, it's fraud," Lethbridge rally organizer Blaine Greenwood told CTV News.

"Our right to vote needs to be protected."

With a report from CTV's Richard Madan and CTV Lethbridge's Alesia Fieldberg