He also arranged meetings with government officials to discuss the Bahrain Independent Commission of Inquiry (BICI) follow-up report.
But Weatherby’s visa application was rejected at Bahrain International Airport and he was told to board the return flight immediately.
Weatherby travelled to Bahrain in 2010 and 2011 to observe trials on behalf of the Bar Human Rights Committee. He is a member of the group’s executive committee.
Almarzooq was arrested in September after he made a speech critical of the government at a political rally attended by nearly 6,000 people near the village of Saar. He was charged with inciting youth violence and trying to overthrow the government.
Amnesty International called for him to be "immediately and unconditionally released".
"His arrest is yet another blow to the National Dialogue which the Bahraini authorities have been flaunting as a reason to cancel the visit of the UN expert on torture to the country. However harsh his speech towards the authorities, he should not have been arrested for expressing his views," said Hassiba Hadj Sahraoui, Middle East and North Africa deputy director at Amnesty International.
Wefaq said the refusal to allow Weatherby into the tiny Gulf island was "unjustified and unacceptable".
"We view this incident as a clear indicator that the authorities have something to hide from the international community," it said.
"Given that he was allowed entry to Bahrain in the past, it is also shows the lack of progress over reform and in fact the deterioration in the human rights situation."
