Firefighters responded to 40 bonfire-related incidents across Northern Ireland overnight, a increase of 21% on the Eleventh Night last year.
In a statement, the Northern Ireland Fire and Rescue Service (NIFRS) said it was an “exceptionally busy” night.
Crews stopped a bonfire spreading to an apartment block near Belfast’s Sandy Row but the heat cracked its windows.
Bonfires are traditionally lit in loyalist areas on 11 July, marking the start of Twelfth of July celebrations.
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Between 18:00 BST on Tuesday and 01:00 BST on Wednesday, NIFRS dealt with a total of 213 emergency calls.
During its busiest period, its regional control centre handled one 999 call every minute.
Assistant chief fire officer Alan Walmsley told BBC Radio Ulster that the two most serious bonfire-related incidents were both in Belfast.
They were at Wellwood Street (off Sandy Row) and at the Bloomfield Walk/Ravenscroft Avenue site in the east of the city.
Mr Walmsley said glazing was badly damaged when the Sandy Row bonfire came “very close” to a ten-storey apartment block in Wellwood Street.
A number of residents who were inside the Victoria Place flats at the time called NIFRS with safety fears.
“All the glazing [on the apartment block] facing the bonfire has been damaged and cracked,” he said.
“If our crews had not been on the scene working so hard, there is no doubt that fire would have spread into that building.”
He added: “We received a number of calls from residents of the flats last night who were extremely concerned because of the heat hitting their building.”
Lucy Ryan, who works for the BBC, was inside the apartment complex at the time.
She told BBC News NI: “We were standing on the first floor, looking out onto the bonfire and saw it all collapse.
“Obviously we felt the heat and we saw the fire was literally coming right over to the building, so we were like ‘right, we have to get out’.
“We just left down the stairwell and as we were leaving to get out, that’s where we saw all the windows were cracked.”
Overall, firefighters were deployed to a total of 133 incidents overnight, which was a rise of nearly 50% on the same period in 2016.
Despite this, the fire service said it “maintained normal service delivery throughout the evening, attending a range of operational incidents including property fires, special service calls and other emergency incidents”.
Mr Walmsley said: “Our crews worked extremely hard last night in extremely punishing conditions and I would just like to place on record my thanks to them.”
He said two fire crews were attacked at they left the scene of a bonfires in Belfast and Larne, County Antrim.
“After working so hard at Wellwood Street for a number of hours to protect properties, as our crews were leaving one of our appliances came under attack by local youths.
“The same situation happened in Larne where our crews worked extremely hard at a bonfire protecting local properties and again when they were leaving they came under attack.”
No injuries have been reported. The assistant chief fire officer said he was disappointed at what were the first attacks on firefighters during Eleventh Night bonfires for five years.
Police said they were investigating complaints about “distasteful” materials placed on some bonfires.
Sinn Fin’s national chairperson Declan Kearney called on unionist politicians to condemn “hatemongers” who put a replica coffin bearing the image of Martin McGuinness, who died in March, on a bonfire in east Belfast.
“I am directly challenging the leaders of all unionist parties to immediately disassociate themselves and their parties from this and other examples of sectarian hate crime,” he said.
Images also emerged on social media of a bonfire in east Belfast draped with a banner carrying a racist message directed at Celtic footballer Scott Sinclair.
In a statement, the PSNI said: “We take hate crime very seriously and actively investigate all incidents reported to us.”
However, many bonfires were held across Northern Ireland without incident on the Eleventh Night.
The annual loyalist tradition of bonfires and parades commemorates the victory of the Dutch-born William III over the Catholic forces of King James II at the Battle of the Boyne in 1690.
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