RNA - Bahrain's public prosecutor charged Sheikh Ali Salman, secretary-general of the al-Wefaq party, and Hassan Sultan, a former member of parliament, of colluding with Qatar to carry out "hostile acts" in Bahrain and damage its "prestige", BNA reported.
"The defendants had received financial support from Qatar for carrying out their activities aimed at harming the status and interests of the kingdom," it said.
It also accused the men of transferring confidential information to Qatar and meeting with officials from the Lebanese armed group Hezbollah.
Salman's wife Alya Radhi wrote on Twitter she had spoken with her husband by phone and he denied all charges.
Salman is already serving a nine-year sentence on charges that include attempting to overthrow the regime and inciting sectarian hatred.
"This new case tries to kill two birds with one stone," Sayed Ahmed Alwadaei, the director of advocacy, at the Bahrain Institute for Rights and Democracy said.
"On the one hand is Sheikh Ali Salman, who the UN has ruled is arbitrarily imprisoned for his political activity as the leader of the opposition, and on the other is Qatar, whose crisis with Bahrain is being used to further punish dissidents.
"These charges are a vindictive low for the Bahraini regime."
Bahrain, along with Saudi Arabia, the United Arab Emirates and Egypt, cut ties with Qatar on June 5 and imposed a land, sea, and air embargo accusing Doha of supporting "terrorism".
Tensions have been running high in Bahrain where there is a growing gap between the minority Sunni-led government and the island's Shiite majority population.
Bahrain's Saudi-backed authorities crushed the Arab Spring protests shortly after they erupted on February 14, 2011, and have refused to listen to opposition demands for reforms.
Al-Wefaq, the main opposition party, was shut down last year after the Ministry of Justice accused it of undermining the state, spreading sectarianism, and having connections to "terrorist" activities.
The party has denied the charges.
Human rights groups have accused Bahrain of clamping down on dissent and violently cracking down on protests.
The government says the protesters are supported and influenced by Iran, but activists insist they are fighting for jobs, housing, and political clout.
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