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A gathering of Egyptian Islamist groups called on Wednesday for mass demonstrations to counter an opposition protest campaign scheduled for the end of the month against the rule of President Mohammed Morsi, and accused a group of prominent businessmen and politicians of plotting violence to destabilize the country.

The news conference, bringing together a Muslim Brotherhood from which Morsi hails, highlights growing political tensions and polarization ahead of June 30, the one-year anniversary of the president's accession to office.

The group did not produce any evidence at the conference to back their far-reaching accusations against two businessmen, the owner of a private sector television station, two parliamentarians, and unnamed Gulf countries. At least one of those named says he planned to sue.

The opposition, including Islamists, criticized the conference as a conspiracy-theorizing, and one spokeswoman said it was a sign of Brotherhood nervousness ahead of June 30.

Many of those named were prominent during the era of Hosni Mubarak, and the accusations fit the narrative — frequently advanced by the government — that the ousted autocrat's loyalists are behind Egypt's current political disorder.

Mubarak loyalists have mostly kept a low profile since Ahmed Shafiq, Mubarak's last prime minister, was defeated by Morsi in 2012 elections.

Since then the most prominent opposition bloc has been the liberal and secular-leaning National Salvation Front, some of whose figures were prominent in the 2011 uprising that toppled Mubarak. No one from the NSF was accused.

Businessman Mohammed el-Amin, the owner of Egyptian CBC TV network, told state-run Al-Ahram that he will sue those who accused him. He called it a "public terrorist threat."

CBC hosts some of the Brotherhood's most prominent critics, including liberal satirist Bassem Youssef, who has been investigated for insulting the president. Morsi and other Brotherhood figures regularly denounce Egyptian media that they claim are biased against them.

Reading out a joint statement at the conference, Nasr Abdel-Salam, head of the Construction and Development Party that is the political arm of the former Gamaa Islamiya militant group, said that his group had obtained information about plans to instigate violence starting June 14.

He said that the businessmen are hiring thugs to set fire on buses that will transport the Brotherhood supporters from the provinces to Cairo for rallies and set fire to offices of Islamist groups across the country. Dozens of Brotherhood offices have been set on fire during previous rounds of unrest.

The group called for a June 21 rally at a mosque near the presidential palace under the slogan, "No to violence." The opposition calls for protests on June 30, the one-year anniversary of Morsi's taking power, to demand his ouster.

Mai Wahba, one of the spokespersons of the anti-Morsi campaign, said that the list of accusations "just shows their fear of June 30. This is what Mubarak regime use to do before the revolution. I see the same fear of that day." She was speaking in a live interview with private-owned ONTV.

One major Islamist party, the ultraconservative Salafi Al-Nour party, did not attend the conference. It put out a separate statement decrying "unprecedented discontent" in the street and among Islamists themselves and criticized talk of conspiracies.

"Such rallying and swearing by the two sides creates a war atmosphere, as though we are going for a confrontation where everyone will lose and there will be no victor," the party said in a statement posted on its official Facebook page. "We are not in a war or a battle between two camps, the camp of faith and the camp of infidels ... This categorization is mistaken."

It called for a national dialogue to avert confrontations.