Nicola Sturgeon has said that today's slow economic growth figures " highlight the need for an alternative to austerity".
Ms Sturgeon called for "modest spending increases" and "investment in actions to get our economy growing faster, to get more people into jobs and to raise our revenues".
UKIP leader Nigel Farage has said that today's slow economic growth figures "will be very disappointing for the government".
He said: "Their whole general election pitch is that there's growth in the economy... I've felt for too long that actually not many people are feeling the benefits."
Liberal Democrats Treasury Secretary Danny Alexander insisted the economy was continuing to make progress but that more needed to be done.
"The British economy is recovering well, but these figures remind us that there is still work to do to secure the recovery," he said.
Responding to today's disappointing economic figures, the shadow Chancellor Ed Balls said that the Conservatives "have not fixed the economy for working families".
He said: “Tory economic policy may be helping a few at the top but for most people bills have gone up faster than wages, which are down £1600 a year since 2010.
"The Tories just don’t understand that Britain only succeeds when working people succeed."
The Chancellor George Osborne has insisted that it's good news that the economy is growing, even though the latest figures are half as strong as at the end of 2014.
ITV News Economics Editor Richard Edgar spoke to George Osborne this morning:
Britain's economy is still growing, but it grew much slower in the first quarter of 2015 than it did at the end of last year.
Official figures show that GDP grew by 0.3% between January and March this year - half as much as it grew between October and December 2014, at 0.6%.
This is the weakest quarterly growth since the end of 2012 - and with the General election just nine days away, this will be seen as a major setback for the government.
The ONS puts the blame on a slower service sector, a shrinking construction industry, and a slight squeeze on industrial production caused by lower oil and gas prices.
ONS chief economist Joe Grice said: "As always, we warn against reading too much into one quarter's figures."
The Prime Minister has said that today's just-released GDP figures show that "our economy is still growing" and that "we can't take the recovery for granted".
GDP figures show our economy is still growing, but we can't take the recovery for granted. Don't risk it with Ed Miliband and the SNP.
Mr Cameron was responding to figures showing slower economic growth in the first quarter of 2015 than in the last three months of 2014.
After the ONS released slower-than-expected figures on Britain's economic growth, George Osborne has insisted that the "future of the economy is on the ballot paper".
GDP figures show future of the economy is on the ballot paper. We should stick to the plan that's delivering a brighter more secure future
Figures show that the economy grew by 0.3% in the first quarter of 2015 - half as much as the 0.6% it grew between October and December 2014
With rising instability abroad, now is worst possible time to vote for instability at home.
The UK economy grew by 0.3% during the first quarter of 2015, the Office for National Statistics said today.