Dissident republicans were behind a gun attack on police officers in the Bogside area of Londonderry overnight, say the PSNI.
An automatic weapon is believed to have been used in the incident during which six shots were fired.
The shots struck the city's walls and nearby trees.
Leaders of the main political parties in Northern Ireland have come together to condemn the recent violence and call for calm.
PSNI Superintendent Gordon McCalmont said an attempted murder investigation has been launched.
"We are working on the basis that automatic gunfire was used," Mr McCalmont said.
Sixteen petrol bombs and five paint bombs were also thrown in the vicinity of the city walls and at police patrols.
"This was a deliberate attempt to kill officers," said Gordon McCalmont.
"I'm aware that a large crowd of people was around the gunmen at the time.
"I'm in no doubt that violent dissident republicans stepped out last night with the intention of killing my colleagues.
"As a community we should be concerned. There were other people on the walls last night too.
"We will not be deterred from our work. We will work to keep people safe."
Just spent inspirational afternoon in Derry/Londonderry - reviewed PSNI resources operating in Derry City & Strabane this afternoon with District Commander & other officers & staff in support functions. 405 officers deployed despite the social media myths of imbalanced policing. pic.twitter.com/ImxtXPQaAj— George Hamilton (@ChiefConPSNI) July 11, 2018
On Wednesday evening, PSNI Chief Constable George Hamilton tweeted that he had reviewed PSNI resources operating in Derry and Strabane and that there were "405 officers deployed despite the social media myths of imbalanced policing".
The Secretary of State for Northern Ireland, Karen Bradley, condemned the shooting.
"Anyone who was in this part of the city, at this time, including children or young people could have been killed," she said.
"This murderous attack has to be condemned by all right thinking individuals."
The political leaders issued a joint statement calling for calm in the area.
"The shots fired last night were a clear and obvious attempt to murder police officers. There must be a strong, clear and united voice against those who would engage in such disgraceful violence.
"As a society we must all stand with those who maintain law and order and who protect all sides of our community.
"We condemn any illegal activity and and urge those who are damaging their own community and attacking and intimidating their neighbours to stop. We would urge people to work with the police to bring those involved in criminality to justice."
Tánaiste (Irish Deputy Prime Minister) Simon Coveney said he was "very concerned at the escalation of violence in Derry".
"The ongoing reckless endangerment of lives, which is being orchestrated by a small and violent minority, must stop," he said.
"It is shocking that young people are being allowed - and even encouraged - to engage in the dangerous activities of the past few nights.
"I would urge all those involved to consider their actions and reflect on the damage that they are wreaking on their city and their community."
'Orchestrated by sinister groups'
Three people have been arrested to date for trouble on Monday night.
A woman and two police officers were injured following disorder on Monday night.
The Bishop of Derry and Raphoe, Rt Rev Ken Good, and the Bishop of Derry, Dr Donal McKeown, paid a joint visit to the Fountain and Bogside on Tuesday afternoon, to meet people who had been affected by the attacks over the past few days.
As the bishops left St Columb's Wells, a van was abandoned at the foot of the Lecky Road flyover and set on fire.
The clergymen were in the midst of their visit, which included speaking with youth workers, residents near the peace wall, as well as senior citizens from Alexander House who have been caught up in recent attacks.
Chairman of the Police Federation, Mark Lindsay, said: "I'm totally disgusted that in 2018 we are still seeing police officers being subjected to murder bids.
"This is clearly being orchestrated by sinister groups and we are very concerned about that.
"Children in the area might think this is the right thing to do. That is not the case."
Independent councillor Paul Gallagher said shots being fired "was wrong" but refused to condemn the incident when asked on BBC Radio Foyle.
"We need dialogue," said Mr Gallagher.
"We can't take this in isolation. Independent councillors have been working hard behind the scenes."