Police have carried out further raids in relation to a major anti-terrorism operation that has resulted in four people being charged so far.

Counter-terrorism police have conducted more raids following the major operations in Sydney and Brisbane that resulted in four people being charged, including one accused of plotting to kidnap and kill members of the public.

Australian Federal Police acting commissioner Andrew Colvin said further search warrants had been executed on Thursday night.

While he would not go into detail, he said the investigation had not been concluded.

A “number of people” were still being detained following the raids in Brisbane and Sydney on Thursday morning.

Prime Minister Tony Abbott said the detainees were people “of interest to our police and security services in connection with terrorist events we believe, on the basis of intelligence, were likely within a few days”.

Mr Abbott said the national security challenge was “more serious today than at any time in the past”.

“It is a serious situation when all you need to do to carry out a terrorist attack is to have a knife, an iPhone and a victim,” he said.

The prime minister said he had been advised in early September of potential threats to “government, government people and Parliament House”.

This triggered a review of security and a decision to put the AFP in charge of internal and external security of the parliament.

“The community expects government at all levels to keep them safe and this government will not let them down.”