Tunisia's powerful UGTT labor union called a general strike in Zarzis on Tuesday, bringing to a head days of smaller protests to demand authorities do more to find missing bodies and improve living conditions.
Thousands of Tunisians demonstrated in the coastal city Tuesday to demand a renewed search for relatives.
"We want to know the truth," local activist Ezzedine Msalem told AFP, denouncing "a state crime perpetrated against the inhabitants of Zarzis".
Protests began in Zarzis this month after a boat believed to be carrying 18 migrants disappeared. Last week local fishermen searching for the wreck found eight bodies.
Anger increased when authorities buried the bodies in a graveyard for migrants rather than work to identify them, and were slow to search for those missing.
President Kais Saied spoke about the Zarzis protests on Monday, using a video of his meeting with Prime Minister Najla Bouden to say the government had tried to identify the missing and would seek to find those responsible for people trafficking.
On Tuesday, Saied asked Justice Minister Leila Jaffel to open an investigation "so that Tunisians can know the full truth and who was behind these tragedies."
The Tunisian Human Rights League said authorities had "not devoted the necessary resources to search and rescue operations in a timely way" and called for an inquiry into the burials.
Dozens have died this year in shipwrecks as boats attempted the journey from Tunisia's eastern coast to the Italian island of Lampedusa.
Meanwhile, opponents of Saied, who regard his recent expansion of powers as an undemocratic coup, which he denies, have protested in their thousands on the streets of central Tunis.