The amendments could make it impossible for members of Islamic groups to seek the Egyptian presidency as they could ban political activities based "on any religious reference or basis".
Under the existing system, the Muslim Brotherhood could field a presidential candidate as an independent if it wins enough seats in parliament and local councils.
"I think now is the time to be all together, all opposition parties ... all to face these imposed amendments of the constitution regardless of their differences," Essam El-Erian, leader of the Muslim Brotherhood, told Al Jazeera.
He said that the opposition parties would not take part in parliamentary discussions about Mubarak's plans to change 34 articles of the constitution.
"Under a dictatorship regime you have an automatic, mechanical majority in the parliament ... now we are going to boycott the parliament during discussions and secondly have a general situation to go to the people of this country to call on them to say 'no' or boycott the elections," El-Erian added.
The revisions are expected to weaken the role of judges in monitoring the conduct of elections, a move the Muslim Brotherhood says could lead to vote rigging.
During the 2005 elections several judges spoke out against the election practices they witnessed.
More than 300 Brotherhood members are in detention, including third-in-command Khairat el-Shatir, who was referred to a military trial with 39 members last month on charges that include terrorism and money laundering.
The constitutional changes could also give the president the right to try anyone before a court of his choice, including before a military tribunal.