The head of the Venezuelan Episcopal Conference, Diego Padrón, said the reforms would turn the country into a "military dictatorship".
Mr Maduro issued a decree in May to establish a popular assembly that will write a new constitution.
The opposition is boycotting the vote to choose the assembly's members.
The election will be held on 30 July.
"This assembly will be imposed by force and its result will be that constitutional status will be given to a military, socialist, Marxist and communist dictatorship," said Monsignor Padrón.
The Venezuelan Episcopal Conference, which gathers the country's bishops, considered Mr Maduro's proposed popular assembly "unnecessary and inconvenient," he added.
He also criticised the attack against members of the opposition-held National Assembly by government supporters on Wednesday.
Several lawmakers were beaten up by intruders brandishing sticks and pipes.
"This attack shows that the government is unwilling to abandon violence," said Mosignor Padrón, who has several times in the past criticised the government of Nicolás Maduro.
Mr Maduro's plans to create a citizen's assembly to rewrite the constitution has been heavily criticised by the opposition, which says it is an attempt by the president to increase his powers by sidestepping the opposition-controlled National Assembly.
Mr Maduro argues that it will "bring peace" to Venezuela, where more than 90 people have been killed in protest-related violence since 1 April.
He has accused the opposition of plotting a right-wing coup against his socialist government.
Venezuela is going through a serious economic crisis, with the shortage of many goods, high inflation and rampant crime.
The opposition accuses Mr Maduro and his predecessor, the late Hugo Chavez, of mismanaging the economy.
Venezuela's Roman Catholic Church has criticised President Nicolás Maduro's decision to have the constitution rewritten.