The ABC has increased security measures at its major Australian offices after threatening phone calls were made to the national broadcaster following a controversial appearance by a former terrorist suspect on one of its programs.

The move to tighten security came after Prime Minster Tony Abbott said on Thursday that the ABC should sack the people responsible for the appearance.

“They compounded the mistake by re-broadcasting the program,” he said on Thursday. “Heads should roll over this.”

• ABC boss: ‘A’ is for ‘Australian’
• Tony Abbott: ‘Heads should roll’ at the ABC
• Waleed Aly rips into Zaky Mallah on The Project

In an email to staff at its Sydney Ultimo Centre, the ABC warned that the building would revert to “after-hours” security arrangements, whereby only staff are allowed entry to the building, Fairfax Media has reported.

“There have been a number of threatening phone calls,” an ABC spokesman told Fairfax Media, confirming new security arrangements.

The spokesman said the ABC received more than 1000 phone calls about the decision to allow the former terror suspect Zaky Mallah onto the Q&A program, describing the security measures as “precautionary”.

On Thursday, Mr Abbott announced he had ordered an “urgent government inquiry” into the live appearance, unsatisfied that the ABC’s internal inquiry into the show would be sufficient.

ABC managing director Mark Scott said on Thursday the ABC would co-operate with the inquiry – and had already launched one of its own into Q&A – and rejected any suggestion the network might be aiding terrorists through some of its editorial decisions.

“The ABC is clearly Australian, it’s on the side of Australia,” Mr Scott told an audience of public affairs specialists in Melbourne on Thursday night.

“The A in ABC is for Australian.”