Sheikh Ali Salman, the head of the influential Al-Wefaq bloc, has remained in custody since authorities arrested him on December 28 on charges including seeking regime change.
Meanwhile, hundreds attended the funeral of a man who died from what Al-Wefaq said may have been tear gas inhalation following Saturday clashes between protesters and police in the Shiite village of Bilad al-Qadim, where Salman lives.
The cause of the death of the man identified as Abdulazziz Salman Alsaeed remain unclear and authorities have not released an official statement. Witnesses said that mourners clashed again with forces after the Sunday funeral.
"When it comes to punishing peaceful critics of the government or ruling family, Bahrain is a serial offender," said HRW's deputy MENA director Joe Stork.
Salman's arrest "seems calculated to send a message to Bahrainis and the world that political reconciliation and respect for fundamental rights is completely off the table," said Stork.
HRW said that authorities have so far "failed" to release evidence against Salman, urging his immediate release and calling for charges against him to be dropped.
Prosecutors allege that Salman confessed to contacting foreign regimes and political groups abroad, some of whom had "expressed readiness" to interfere in Bahrain's affairs.
Iran, which condemned Salman's arrest, has been accused of interfering in Bahrain since its Sunni ruling family crushed month-long protests led by Al-Wefaq in 2011 seeking an elected government in the Shiite-majority kingdom.
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"Bahrain's allies in London, Paris, and elsewhere have largely stayed silent as Bahrain has filled its jails with the people who hold the key to the political solution the (United Kingdom) UK and (United States) US claim to support," Stork said.
"Who will Bahrain have to detain and on what nonsensical charges before Bahrain's allies speak out?"
Bahrain, home to the US Fifth Fleet, is among the countries that have joined the international coalition against the Islamic State group in Syria and Iraq.
Prominent Shiite activist Nabeel Rajab will stand trial on Tuesday over tweets deemed insulting to public institutions, in an another case that has been criticised by rights groups.
The United States has expressed concern over Salman's detention, warning it could only inflame persistent violence that has gripped the kingdom since 2011.
Bilad al-Qadim has seen nearly daily protests since Salman was arrested.
At least 89 people have been killed in clashes with security forces since 2011.
Hundreds have been arrested and put on trial, human rights groups say.