Germany's chancellor, Angela Merkel, led European Union critics of US and British attacks on their plans on Friday to resolve the sovereign debt and banking crises.

Merkel criticised both Barack Obama and David Cameron for opposing EU proposals for a financial transaction levy, or Tobin tax, and demanding "big bang" solutions.

Spain's economy minister, Elena Salgado, smarting from a cut in the country's credit rating from Standard & Poor's, insisted it would meet its tough deficit target and accused rating agencies of being too swayed by exaggerated eurozone problems.

Merkel, speaking at a conference of the engineering trade union IG Metall in Karlsruhe, said: "It cannot be that those outside the eurozone who press us again and again for comprehensive action are, at the same time, comprehensively working together to prevent the introduction of a financial transaction tax."

She added: "This is out of order. We must ensure that financial market actors share in the costs of fighting the crisis. I will push for this until it happens, at least in Europe but preferably worldwide."

Desmond Tutu, archbishop emeritus of Cape Town, has added his voice to those calling for the transaction tax – also known as a Robin Hood tax – and urges world leaders to ensure that the revenues go to the world's poorest people. In a letter to Saturday's Guardian, Tutu says: "The G20 has a chance to show it can assume the mantle of global economic leadership in these difficult times. Backing a financial transaction tax to help the world's poorest would show our leaders are ready to rise to the challenge."

Max Lawson, Oxfam's head of policy, said: "George Osborne should ditch his defence of City casino capitalism and throw his weight behind G20 countries pushing for a Robin Hood tax."

Accused by Obama, Cameron and others of a lack of leadership, Merkel defended her cautious approach to the sovereign debt crisis, arguing that there was no overnight solution. She accused some investors of being almost "euphoric" at the prospect of a Greek default being agreed at the EU summit on 23 October, saying the debt crisis needed to be approached with great caution.

"The solution of the debt crisis is a long, exhaustive process, one of many steps and measures which must be sought and examined so that they bring more advantages than disadvantages when they are decided upon," she said.

She added: "Eurobonds are no miracle solution. They will not help us in the current circumstances." Her comments came as S&P downgraded Spain a notch from AA to AA minus in a move echoing that of its fellow US ratings agency Fitch last week.

It said: "Despite signs of resilience in economic performance in 2011, we see heightened risks to Spain's growth prospects due to high unemployment, tighter financial conditions, the high level of private sector debt and likely economic slowdown in Spain's main trading partners."

It said labour market reforms were "incomplete" and there was the likelihood of further asset deterioration for Spanish banks. It shaved its forecast for overall growth in 2012 closer to 1% from 1.5%.

Salgado, whose socialist government is expected to lose general elections on 20 November, said it was on course to achieve its deficit reduction target thanks to proceeds from a wireless spectrum auction and lower interest payments.Spain is viewed in Brussels as having moved swiftly to recapitalise its banks, and Salgado insisted that plans adopted at next week's summits would spur resumed growth across Europe.