Iranians have started forming human chains around power plants after Donald Trump promised "a whole civilisation will die tonight".
The US President has given Iran's leaders until 1am to end its blockade of Gulf oil or see the US destroy every bridge and power plant in Iran.
Iranian state media has released photographs showing crowds gathering around some bridges and power plants in linked chains and holding up flags of the Islamic Republic.
An Iranian official had earlier called on people to stand hand in hand to say: "Attacking public infrastructure is a war crime."
Meanwhile, Iran’s president, Masoud Pezeshkian, said 14 million people had signed up in a voluntary drive to fight for their country and "declared their readiness to sacrifice their lives in defence of Iran".
As the clock ticks down, strikes on Iran intensified, hitting railway and road bridges, an airport and a petrochemical plant.
US forces attacked targets on Kharg Island, home to Iran's main oil export terminal.
Iran responded by declaring it would no longer hold back from hitting its Gulf neighbours' infrastructure and said it had carried out fresh strikes on a ship in the Gulf and a huge Saudi petrochemical complex.
Mr Trump has criticised the human chains, telling NBC News they were "Totally illegal. They’re not allowed to do that."
Flags of the regime were raised on bridges
|FARS NEWS
Mr Trump is aware of Pakistan's proposal for a two-week extension to a deadline he imposed on Iran, and a response will come, White House press secretary Karoline Leavitt confirmed.
The Pakistani President Shehbaz Sharif said: "Diplomatic efforts for peaceful settlement of the ongoing war in the Middle East are progressing steadily, strongly and powerfully with the potential to lead to substantive results in near future. To allow diplomacy to run its course, I earnestly request President Trump to extend the deadline for two weeks.
"Pakistan, in all sincerity, requests the Iranian brothers to open Strait of Hormuz for a corresponding period of two weeks as a goodwill gesture.
"We also urge all warring parties to observe a ceasefire everywhere for two weeks to allow diplomacy to achieve conclusive termination of war, in the interest of long-term peace and stability in the region."
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Shehbaz Sharif has called on his US counterpart to extend the deadline
|GETTY
A Tehran official told Reuters Iran is simultaneously ready for peace and war.
Explosions were heard in Doha late on Tuesday night, according to an eyewitness in the Qatari capital.
Iran's UN ambassador, Amir Saeid Iravani, called Mr Trump's threat "deeply irresponsible" and "profoundly alarming".
Speaking at a UN Security Council meeting, where China and Russia vetoed a resolution aimed at protecting commercial shipping through the Strait of Hormuz, Mr Iravani said Mr Trump's "rhetoric is unfitting of any political leader".
A view of Tehran, amid the US-Israeli conflict with Iran, in Tehran
|REUTERS
US Secretary of State, Marco Rubio, has spoken with Foreign Secretary Yvette Cooper about the escalating Middle East crisis.
US State Department principal deputy spokesman Tommy Pigott said: “Secretary of State Marco Rubio spoke today with UK Foreign Secretary Yvette Cooper about the Iranian regime’s ongoing attacks across the Middle East and the critical importance of restoring freedom of navigation in the Strait of Hormuz.
“The secretary and Foreign Secretary agreed on the need for international efforts to ensure shipping can move freely and energy supplies can reach global markets.”
The Israeli military said there was a possibility of an increase of fire towards Israel as President Trump's ultimatum nears. It said it was ready to operate both "defensively and offensively".