A prominent Bahraini human rights activist was taken into custody this week over a tweet that was allegedly "insulting" to the country's Ministry of Interior. Rights advocates say the detention is unlawful and an example of the ongoing suppression of freedom of expression and dissent in the Persian Gulf country.

The Observatory for the Protection of Human Rights Defenders (OPHRD) has called for the immediate release of Nabeel Rajab, President of the Bahrain Centre for Human Rights (BCHR), who was detained Wednesday for comments he posted to the social media site last month.

Rajab had just returned from a two-month overseas advocacy tour. In the hours before his detention, he tweeted that the criminal investigations department had called for him to go in for questioning, but that he was resisting until he received a formal written notice.

I received a call from the Criminal Investigation dep to go immediately to Cyber Crimes Dep. waiting for written summon in order to go. — Nabeel Rajab (@NABEELRAJAB)October 1, 2014

Bahrain's Interior Ministry later confirmed Rajab was summonsed "regarding Tweets posted on his Twitter account that denigrated government institutions."

The day after Rajab's arrest, his associates began posting updates on the situation to his Twitter account, confirming his week-long detention. The OPHRD noted that lawful detention is only permitted in Bahrain for up to 48 hours before a person must be released or prosecuted. Rajab has been imprisoned more than twice that long.

The Public Prosecution has ordered the detention of Nabeel for 7 days on pending investigation.— Nabeel Rajab (@NABEELRAJAB)October 2, 2014

Advocates believe that Rajab has been unfairly targeted because of his high-profile pro-democracy political activism, including his role in organizing Shiite-led opposition protests demanding democratic reforms in Bahrain that formed part of the Arab Spring uprisings across the Middle East in 2011.

Although the protests did not topple the government in the US-allied country as they did in neighboring Arab nations, the political leadership and opposition have faced a tense standoff ever since.

The ongoing tensions continued last month, when Shiite opposition protestors marched down Manama's Budaiya Highway following Crown Prince Sheikh Salman bin Hamad Al Khalifa's announcement of limited political and judicial reforms.

Local media reported that protesters took to the streets again Saturday in several towns, "demanding a referendum and the release of all political prisoners."

For his efforts in the pro-democracy movement, Rajab was convicted of organizing illegal demonstrations and served two-years (reduced from a three-year sentence on appeal) in prison until he was released in May.

Since then, the OPHRD claims Rajab has been subject to "arbitrary detention and ongoing judicial harassment." He is currently being investigated under article 216 of the Penal Code which makes it illegal to, "offend by any method of expression the National Assembly, or other constitutional institutions, the army, law courts, authorities or government agencies."

If convicted, Rajab could face up to three years more in prison, the group said.

This week the UN human rights office also called for Rajab's immediate release, saying his detention sends "a chilling message to other activists of the consequences they may face for any criticism of authorities."

Spokesman Rupert Colville also said Friday the UN welcomed the release of fellow activist Maryam al-Khawaja who was detained last month for allegedly assaulting two police officers. Khawaja's trial has been set for Nov. 5, according to the Associated Press.

"Human rights defenders in Bahrain must be able to carry out their work without fear of reprisals," Colville said.

The allegedly offending tweet posted by Rajab called Bahrain's security institutions "ideological incubators." It was posted in response to a video released by Islamic State militants depicting some Bahraini recruits, a few of whom had previously worked at Bahrain's Ministry of Interior, according to an Amnesty International statement.