A pipe bomb has been thrown at a police patrol in Derry, a senior officer said.
A number of homes were evacuated during the security alert at Leckey Road flyover close to a controversial nationalist bonfire built in the middle of the main road.
PSNI district commander Mark McEwan said: "We are investigating all offences committed at the bonfire in the Bogside last night including a serious incident where a viable pipe bomb-type device was thrown at a police patrol."
The six metre-high blaze had partly blocked Leckey Road in the Bogside. Sinn Féin election posters and Union flags were burned.
An attempt to move the fire from the middle of the road had failed and Mr McEwan said police were just one part of a multi-agency approach to resolving issues around bonfires.
Sinn Féin Stormont assembly member Raymond McCartney blamed dissident republicans.
"Several families, including elderly people, had to be evacuated from their homes due to a security alert on Charlotte Street.
"This situation was dictated by a tiny group of young people who are being exploited by more sinister elements who are clearly determined to act against the wishes of the local community."
He said the scenes around the bonfire were in stark contrast with the excellent finale to this year's Féile which showed the positive community spirit of the people in the area who want nothing to do with the bonfire.
"There was an opportunity to avoid all of these problems through dialogue. Unfortunately that failed because people absented themselves from that process or effectively scuppered it by acting as cheerleaders for anti-community behaviour.
"That process was an opportunity lost and we need to see renewed efforts to avoid a repeat of this situation in the future so the local residents can live in peace."
Foyle DUP Assembly member Gary Middleton said the bomber had put dozens of lives at risk.
"Those who constructed and threw this device are terrorists who clearly have no regard for the lives of anyone in the city."
He added: "We have witnessed too many occasions where such terrorists have held this city to ransom and we must demonstrate that such violence and threat cannot be part of our society."