Jamie Dimon, chief executive of the Wall Street bank JP Morgan, has made a spirited defence of the global financial sector, insisting many banks have been "ports in the storm" since 2008, and criticising regulators' response as "five years of scapegoating".

Dimon has come under fire in recent months over the $6bn (£3.8bn) losses run up by the so-called London Whale, a team of investors in the London office of its chief financial officer. Dimon had his bonus cut in half, to $11.5m, because of the losses but escaped harsh criticism from the bank's internal reports into the London trading fiasco.

In a debate that became heated at times at the World Economic Forum in Davos, Switzerland, Dimon rejected the argument that banks remain too big and too complex to be safe.

"JP Morgan was not a fairweather friend: we were there in good times and bad times for everyone, including nations," he said, singling out the bank's role in lending to the embattled governments of Spain and Italy.

He was fiercely critical of regulators' response to the crisis, saying: "We've had five years of scapegoating and pointing fingers, but we don't have a better system yet." In the US, he said there were too many regulators, all trying to do similar jobs.

Paul Singer, the controversial hedge fund manager from Elliott Associates, which recently took the Argentinian government to court in the US over its 2001 debt default, insisted that banks' trading positions were still too difficult for outside analysts, including regulators, to understand.

"I don't think the banking sector is too big; I think it's too leveraged and too opaque," he said, warning of "this combination of classic banking activities, which are underwritten by the deposit guarantees by the governments, and trading activities".

Min Zhu, deputy managing director of the International Monetary Fund, also appearing on the panel, argued that the financial sector remained too big. While banks' lending had been reined in since the crisis, he said, there had been "zero deleveraging" of derivatives – the complex bets banks make with each other.

He said the "shadow banking" sector, which includes hedge funds and private equity firms, should not be allowed to slip beyond the tentacles of regulators.

"We've seen a lot of activity move away from the banks, to the capital markets," Zhu said. "Both the banks and the shadow banks should have a proper regulatory framework to govern them."

But Dimon insisted the burgeoning of complex financial products was just a reflection of modern society. "Finance is a critical part of how the economy functions," he said. "If you're in a barter economy, there are no financial assets. Once a society starts to save, there are financial assets, and you want there to be financial assets."

Axel Weber, chairman of the Swiss bank UBS and a former European Central Bank chief economist, called for a single regulatory framework to cover all global financial institutions, saying: "The expectation was for a level playing field for banks; this has not happened."