The Electoral Amendment Bill is a "flawed document" which parliament should not pass until proper consultations have been made, poll observer groups have said.
The Bill controversially sailed through the Senate on Thursday, despite protestations from opposition MDC-T lawmakers who had staged a walk-out the day before.
MDC-T Senator Morgan Komichi later said the walk-out was a reaction to the "speed with which the ZANU PF administration was bulldozing the Bill through the House without taking their proposals on board.
On Friday, two civil society groups described the Electoral Bill as unconstitutional and urged parliamentarians to block it until the public have been consulted.
The Zimbabwe Election Support Network (ZESN) said it was concerned that "this important piece of legislation that will inform future elections" had been "hurried" through Senate without involving interested Zimbabweans.
"Adequate consultations with the public and other stakeholders are vital to ensure comprehensive alignment of the Bill with the new Constitution," ZESN said in a statement.
The Election Resource Centre also noted several "deficiencies" in the Bill, which would "render it unable to positively influence a change in the architecture of electoral management in Zimbabwe".
The two civic groups note that the Electoral Bill is vague on what constitutes voter education, and the Zimbabwe Electoral Commission (ZEC) remains answerable to the justice ministry, in violation of the constitution.
Another concern is that the Bill denies the right to vote to Diaspora-based Zimbabweans, or those in hospital or prison.
In its post-poll review conference held last month, ZEC also proposed changes to the conduct of elections in the country but these, too have been left out of the Bill.
The head of Election Resource Centre, Tawanda Chimhini, said they have launched a "Why Rush - Give Us Time" petition to try and put pressure on the justice ministry to reconsider.
"The Electoral Bill does not address the numerous challenges which were noted by various observer missions during the 2013 elections and acknowledged by ZEC.
"The Bill is weak and has unacceptable errors that betray little seriousness on the part of the drafters," Chimhini said.
"In its current form the Bill will not take Zimbabwe forward in terms of electoral reforms and if it is passed we intend to challenge its constitutionality in court."
Chimhini said to avoid passing into law such "flawed documents" the public should be allowed to participate and have a say in any Bill that is brought before parliament, in accordance with the constitution.
"The problem with this Bill, as with all our electoral laws, is that it weakens rather than strengthens systems and as long as these are allowed to exist, we won't have credible or acceptable elections in this country," Chimhini added.
He said they hoped that MPs from both sides of the political divide will show the same unity and energy in rejecting the Bill just as they have done on corruption.
The Electoral Bill is one of several other pieces of legislation that have to be aligned with the country's new constitution which was adopted last year.
Hopes were high that the new charter, campaigned for by the three political parties in the unity government which ended last year, would usher in a new era of constitutionality in Zimbabwe.
The constitution limits the president's stay in power. The charter also provides for security sector and media reforms and creates institutions that (in theory) are supposed to promote democracy, peace, transparency and accountability.
But there is no specified timeframe within which old laws should be aligned with the constitution, a loophole that ZANU PF is taking advantage of.
Without the called-for changes and reforms, there is real fear that ZANU PF will continue using these institutions to rig and manipulate the electoral process, just as it did in 2013.