Australian police are searching five properties in Sydney over a suspected terrorism plot to bring down a plane.
Four men were arrested in raids across the city late on Saturday.
Police said they had seized materials that could have been used to make an explosive device. Local media said the plot may have involved a meat mincer.
The men can be held for seven days without charge after a magistrate granted police special counter-terrorism powers.
The suspects are reported to include a father and his adult son.
Authorities have increased security measures at Australian airports, prompting lengthy queues and passenger confusion.
What was the alleged plot?
Australian Federal Police Commissioner Andrew Colvin said the men arrested were allegedly linked to an Islamist-inspired plan to detonate an improvised explosive device (IED).
He said police did not yet have information on "the specific attack, the location, date or time".
Senior government minister Peter Dutton would not comment on reports that the suspected plot involved concealing a device in a kitchen meat grinder, or an allegation in The Australian newspaper about the possible use of poisonous gas.
"I do not want to go into the detail, but... there was a significant threat that [police and intelligence officials] dealt with and are in the process of dealing with," said Mr Dutton, who will soon oversee all of Australia's domestic security agencies.
Who is under arrest?
The four men were arrested in raids in the Sydney suburbs of Surry Hills, Lakemba, Wiley Park and Punchbowl.
According to local media, they include a father and his son and another pair who are also related.
No charges have been laid. On Sunday, a magistrate gave permission for an additional period of detention, meaning the four can be held for up to seven days without charge.
What are police doing now?
They are gathering evidence, including from the houses in Sydney, in a process that could last for days.
Police have said they intervened early because it was a counter-terrorism operation. Had it been another type of investigation, they may have waited before conducting raids.
Mr Colvin urged the public to be patient because police did not yet "have all the pieces of the puzzle to put together".
Prime Minister Malcolm Turnbull praised authorities for moving swiftly and said Australians should feel confident in their security agencies.
"This is now the 13th terrorist plot which has been disrupted by our agencies since 2014," he said.
Australia's national terror threat level remains at "probable" - the third level on a scale of five.
What is the impact on travel?
Passengers have been warned to arrive at Australian airports an hour earlier than usual amid heightened security arrangements.
The new measures have led to huge queues, particularly at the busiest airports in Sydney and Melbourne.
Mr Turnbull said the arrangements could remain for some time.
"I want to thank the travelling public for their forbearance," Mr Turnbull said.