The PSNI say five men have been arrested and a number of items seized after an illegal parade and violent disorder in Co Derry.
Police are investigating the incidents which took place on Easter Monday.
The men - aged 29, 31, 42 ,45 and 50 - were detained following a search at premises in Chamberlain Street in the city on Wednesday morning.
PSNI Chief Inspector Paul McCracken said: "Immediately after the parade and the attack on local police officers on Monday, we had said that robust action would be taken against those who orchestrated the violence and those who participated.
"These searches and arrests are the first phase of that action and more will follow in coming days.
"Those who seek to attack police and bring violence onto the streets can expect to be held accountable for their actions."
Speaking on Monday, Chief Inspector Ivor Morton said: "We attempted to engage with the organisers of the parade over a period of time in a bid to facilitate their event within the law.
"The organisers declined to enter into any discussions with police on this matter.
"The organisers also stated in the run-up to (Monday's) event they were informed by police the parade would be stopped and would not be allowed to go ahead.
Police attacked in Derry/Londonderry. pic.twitter.com/KPRB3eR8X2
— PSNI (@PoliceServiceNI) April 2, 2018
"This was patently untrue and can only be viewed as an attempt by those behind the parade as a bid to raise tensions in the area.
"It is, therefore, regrettable that in raising those tensions a small number of people saw fit to attack police officers with petrol bombs in what was clearly a pre-meditated attack.
"I want to be clear that for anyone who engaged in actions that were unlawful or detrimental to community safety, there will be consequences through the criminal justice system."
Police have been reviewing video footage of the event.