First Minister Carwyn Jones has said the British steel industry cannot be allowed to die, as assembly members reconvene to discuss the Tata crisis.
Assembly election campaigning has been suspended for the crisis recall after Tata Steel UK went up for sale.
About 6,000 Welsh jobs are under threat, including more than 4,000 in Port Talbot.
The assembly meeting comes after tycoon Sanjeev Gupta was linked to a potential purchase of the plant.
Opening the assembly session on Monday, Mr Jones said: "I have one simple message for the people of Wales and the UK government - these plants cannot close."
The first minister said it emerged at the weekend that it was not the European Union holding the UK back, "but the other way around".
Mr Jones said he would support UK government ownership if necessary while a buyer was being found.
AMs banged their desks in support when he ended his statement by saying he wanted the assembly to tell steelworkers: "We stand beside you, shoulder to shoulder."
Responding for the Welsh Conservatives, leader Andrew RT Davies said it was "vital" that domestic steelmaking capacity was kept, while acknowledging "international whirlwinds" surrounding the industry.
He added that the proposed M4 relief road in south Wales "could contribute demand for Welsh steel".
Plaid Cymru leader Leanne Wood said there were "serious questions for the Welsh Government to answer", accusing ministers of being slow to act.
She said nothing should be off the table, including temporary nationalisation, a joint venture or a permanent public stake.
Mr Jones responded by saying a joint venture should be considered but it would need UK government involvement.
He added that Tata had at no time told the Welsh Government it would close its plants.
During the session the first minister confirmed a package of support worth more than £60m that had been available for Tata "remains on the table".
It was made up of a commercial loan plus cash for environmental improvements and for skills and training.
Welsh Liberal Democrat leader Kirsty Williams spoke of a need "to see what steps we can take to make the plants as attractive" for a buyer as possible.
The industry was "too good to let fail", she added.
The meeting was being held in the old assembly chamber in the Ty Hywel building in Cardiff Bay, as the current Senedd chamber is being refurbished.
Dozens of protesters gathered outside the Welsh Assembly buildings in Cardiff Bay ahead of the meeting.
Steelworker and Unite union rep Matthew Pearcy said the recalling of the assembly was a "really positive sign" which gave a "glimmer of hope".
But he added: "I think the Welsh Assembly are limited on what they can achieve.
"It's really down to the UK government to stand up and show us that they mean the words that they have said."
Welsh Secretary Alun Cairns said the UK government "stands ready and waiting to support any potential investor" with the cost of taking on the Port Talbot works, such as pensions and power bills.
Following a steel industry taskforce meeting in Cardiff Bay on Monday, he said: "There are no guarantees on anything in relation to this. We will do whatever we can."
Mr Cairns said he could not disclose how much public money could be involved in the middle of commercially confidential negotiations, but confirmed he had spoken to Mr Gupta about a possible takeover.
The Welsh Government's Economy Minister Edwina Hart said she wanted to avoid "fragmenting" the steel industry, with Tata's British assets being "cherry picked".
UKIP Wales leader Nathan Gill called for the UK government to set up an emergency task force to cut energy costs for industry.
But he claimed European Union rules restricted the scope of UK and Welsh ministers to help the steel industry, and accused the first minister of "political grandstanding".
"It's all very well Carwyn Jones calling an emergency meeting of the assembly and caterwauling about public ownership, but simply passing the plant into public ownership does not solve any of the problems," he said.
"All it does is instead pass the culpability of the £1m loss per week onto the taxpayer, from Tata Steel."
Tata has other Welsh plants at Trostre, Shotton, Llanwern and Newport, while UK plants in Rotherham, Scunthorpe and Corby are also affected.
Trade unions representing Tata Steel workers across the UK have called on the prime minister to take personal charge of the rescue talks.
Union officials from the threatened plants meeting in London accused Business Secretary Sajid Javid of "taking his eye off the ball", and called on David Cameron "to get a grip" of the negotiations.