At least 240 people were charged with treason after the protests, but the president later sought to ease tension and asked prosecutors to "show leniency". Many of those charged have reportedly already been released.
Opposition leader Tundu Lissu was charged with treason in April and remains behind bars.
In a joint statement on Friday, the coalition of Western embassies in Tanzania urged President Samia's government to free political prisoners and allow detainees to get legal and medical help.
"We call on the authorities to urgently release all the bodies of the dead to their families," added the statement issued by the European Union delegation, several EU member countries, Canada, Norway and Switzerland.
They welcomed the government's decision to investigate the causes and circumstances of the violence but stressed that the probe be independent, transparent, and inclusive, involving civil society, faith-based organisations, and political stakeholders.
"We reiterate our call on the government to uphold its international commitments to protecting fundamental freedoms and the constitutional rights of all Tanzanians to access information and express themselves freely," they added.
The statement comes amid growing diplomatic pressure on Tanzania over reported human rights abuses during the disputed election.
UN human rights office spokesman Seif Magango on Friday told a press briefing in Geneva that the Tanzanian government had "intensified a crackdown against opponents since the vote".
"Reports indicate that since mid-November, dozens of individuals including academics, civil society actors and local political leaders have been detained, with several arrests allegedly carried out by unidentified armed personnel," Magango added.
He said hundreds were reportedly killed and more than 2,000 detained after demonstrations broke out following the elections.
"We call for the immediate and unconditional release of all those arbitrarily detained."
The US government on Thursday said it was reconsidering relations with Tanzania in the wake of its heavy-hand response against protesters.
The State Department said the US "cannot overlook actions that jeopardize the safety of our citizens, or the security and stability of the region".
In a defiant speech on Tuesday, President Samia defended the use of force as "necessary" against protesters who "were ready to overthrow the government".
She repeated accusations that unnamed foreign actors had colluded with opposition groups to cause "regime change," without providing evidence to back up the claim.
Samia said her government was ready to confront protesters in fresh demonstrations planned for Tuesday next week.
The 65-year-old leader came into office in 2021 as Tanzania's first female president following the death of President John Magufuli.
She was initially praised for easing political repression, but the political space has since narrowed.