LONDON -- British Prime Minister David Cameron admitted Saturday that he bungled his admission of his investment in an offshore fund revealed in the mammoth data breach of a Panama law firm.

Cameron drew laughs and applause from a sympathetic audience of Conservative Party activists as he opened his speech with a mea culpa.

"Well, it not been a great week," he deadpanned. "I could have handled this better. I know there are lessons to learn, and I will learn them. And don't blame No. 10 Downing Street or nameless advisers. Blame me."

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It was Cameron's first public appearance since his admission Thursday night that he had owned shares in a Bahamas-based trust from 1997 to 2010. He had sidestepped persistent questions on the issue for four days with a string of obfuscating statements issued through aides.

Cameron faces mounting pressure from opposition lawmakers to reveal the full extent of his past investment in offshore investments, particularly those run by his late father, Ian, a millionaire stockbroker who placed much off his savings in trusts based in island tax havens. Labour Party leader Jeremy Corbyn and others say they will press Cameron on the issue when Parliament reconvenes Monday.

A few hundred left-wing protesters banged drums and whistled as they marched Saturday down to Whitehall, the government center, past Cameron's Downing Street residence. Many donned placards denouncing Cameron as a wealthy elitist and one held aloft a pinata of a pink pig with Cameron's face on it.

The British prime minister is one of scores of political leaders, celebrities and sports stars who have been linked to shell companies and investment trusts organized by the Panama City-based law firm Mossack Fonseca, which specializes in registering offshore companies. Cameron has made the closure of global tax loopholes a focal point of his government - and rejected charges of hypocrisy on the issue.

"This government that I lead will go on very clearly, very doggedly, very determinedly making sure that we crack down on tax evasion and aggressive tax avoidance," he said.

Last weekend's leak of 11.5 million documents from four decades of Mossack Fonseca files -- dubbed the "Panama Papers" -- identified Cameron's late father, Ian, as a client.

Cameron admitted in a Thursday television interview that he owned shares in his father's Bahama trust from 1997 to 2010, but sold that stake for a 19,000 pound ($30,500) profit along with all his other unspecified shareholdings shortly before becoming prime minister that year. His father died shortly after he gained office.

Michael Hudson is one of the reporters who has been studying the documents for months.

"The paperwork said clear that they wanted it structured in a way that the profits from these investments would not be taxed in the UK," he told CBS News.

In Saturday's speech, Cameron reiterated plans to make public his recent tax returns to show that he paid all legally due taxes on that Bahamas-based investment. He has declined to answer questions on whether he made other offshore investments.

Labour lawmakers already have demanded that Cameron publish details of all his investment holdings since he became Conservative leader in 2005.

Corbyn said he would publish his own tax returns "very soon, and there'll be no surprises there."