SNP MPs can "guarantee" that support will be delivered to the oil and gas industry by holding the Westminster parties to account, according to Nicola Sturgeon.

Previous UK governments have "played fast and loose" with the sector, Scotland's First Minister said, as she campaigned in Aberdeen ahead of a second televised election debate in two days.

She pledged that "SNP MPs at Westminster will speak as strongly for the North East in Westminster as we have always done at Holyrood".

The SNP leader joined Kirsty Blackman and Callum McCaig, the party's respective candidates for Aberdeen North and Aberdeen South, on the campaign trail in the Castlegate area.

"The North East of Scotland has been let down by Labour and the Tories for generations - but with austerity continuing to bite, now more than ever the area needs a strong voice at Westminster to truly stand up for their interests," Ms Sturgeon said.

"I am proud of the SNP's strong record of standing up for Aberdeen and the North East - delivering on infrastructure projects like the AWPR, supporting small business, playing a key role in supporting the oil and gas sector through the Energy Jobs Taskforce and providing funding for the Oil and Gas Innovation Centre.

"I pledge today that SNP MPs at Westminster will speak as strongly for the North East in Westminster as we have always done at Holyrood."

The oil industry has been calling for tax cuts and further investment amid falling prices and job losses.

Ms Sturgeon added: "Westminster has played fast and loose with our oil revenue and mismanaged the North Sea industry for decades - with tax grabs from successive chancellors holding the industry back and putting jobs and investment at risk.

"The UK Government's delayed response to the current situation in the North Sea proves beyond doubt it is time for a new approach.

"With the right support, the North Sea has a bright future - and with a strong team of SNP MPs holding the Westminster establishment to account, we can guarantee that support is delivered and the interests of the North East simply can't be ignored in the House of Commons any longer."

Ms Sturgeon, Scottish Labour leader Jim Murphy, Ruth Davidson from the Scottish Conservatives and Willie Rennie of the Scottish Liberal Democrats will face each other for the second time in 24 hours later today.

Unlike the clash in Edinburgh last night, tonight's debate will include Scottish Green Party co-convener Patrick Harvie and David Coburn, the Ukip MEP for Scotland.

Mr Murphy was also in the city ahead of the debate for a visit to youth homelessness and unemployment charity Aberdeen Foyer with the party's Aberdeen South candidate Ann Begg.

He warned that the Conservatives will cut more money from Scotland's welfare budget than it currently spends on disability benefit.

He said: ''We have had five years of brutal Tory austerity - but the reality is we have not seen the worst of their plans yet.

''They are planning deeper cuts in the next three years than the last five.

''The Tories want to cut £12 billion from welfare after the election. That's £1 billion from Scottish people - more than all the disability benefits paid in Scotland."

Elsewhere on the campaign trail, Deputy First Minister John Swinney was in Perth to meet the owner and staff of marketing company and living wage employer Volpa, where he will launch the SNP's five-point plan for fair work.

Conservative MSP Annabel Goldie will be joined by party activists at a street stall outside House of Fraser in Glasgow's Buchanan Street.

Former Liberal Democrat leader Sir Menzies Campbell and the party's East Dunbartonshire candidate Jo Swinson will be in Bearsden to meet researchers at the Beatson Institute for Cancer Research to discuss Liberal Democrat plans to protect investment in science and research.

Ms Sturgeon was heckled by a lone unionist protester as she campaigned in Aberdeen, in the shadow of the Gordon Highlanders monument in the Castlegate.

The monument bears an inscription from wartime Conservative prime minister minister Winston Churchill stating that the Highlanders are "the finest regiment in the world".

The protester jostled with security men and SNP activists, who repeatedly cheered to drown out his unionist chants of "UK OK" and "Nicola Sturgeon, don't rip up our flag".

Ms Sturgeon reignited the independence debate last night by refusing to rule out proposing another referendum in the SNP's 2016 Holyrood election manifesto.

She reiterated her message on the streets of Aberdeen today.

"If people decide that there shouldn't be another referendum there won't be another referendum, regardless of what I say or any other politician says," she said.

"I will consider these things when I am writing our manifesto, where things are in issue, right now I am focusing on the 2015 General Election."

"I'm not planning another referendum.

"I was simply making the point last night that I am not going start writing the 2016 manifesto before we have had the 2015 election.

"For there to be another referendum, firstly there would have to be evidence of a change of circumstances and a change in public opinion, and then it would have to be in a manifesto that people would have to vote for."

She added: "I take the view that the people are in charge of these things.

"I cannot force a referendum on the people against their will. It's entirely up to the Scottish people.

"Politicians don't decide these things. Politicians can decide whether to propose them, but it is ultimately the Scottish people who decide.

"The people are in charge - that is one of the big messages of the referendum."