The group of about 20 delegates from the capital Kabul is headed by Abd al-Hafiz Mansur, a former anti-Soviet mujahidin fighter and head of the opposition bloc at the grand assembly that opposed many of the arguments put forward by the government of President Hamid Karzai.
"I myself have discovered more than 15 changes that the government does not have the authority to make," Mansur said.
The new constitution was approved unanimously by the 502 delegates to the loya jirga on 4 January after three weeks of intense debate and finally signed into law by Karzai on Monday.
It enshrines a presidential system of government backed up by a strong bicameral parliament and paves the way for Afghanistan's first democratic elections later this year.
The opposition bloc, which also includes loya jirga delegates from Kabul's surrounding provinces, intends to voice its opposition at a press conference in the capital on Wednesday.
"I myself have discovered more than 15 changes that the government does not have the authority to make"
Abd al-Hafiz Mansur,
Head of opposition bloc
The group will also present a letter expressing their complaints to the US embassy, the United Nations Assistance Mission in Afghanistan, the European Union and Afghanistan's former king Muhammad Zahir Shah.
"The constitution which was signed by President Karzai, if it is
carefully read... compared to the constitution approved and ratified by delegates to the loya jirga has changes," Mansur told AFP on Tuesday.
"After the end of the loya jirga nobody has the authority to change what is in the constitution, but now in the constitution after the loya jirga there has been lots of changes.
The director of the constitutional commission's secretariat Faruk Wardak said the text approved on Monday by Karzai was the final text as accepted by the loya jirga on 4 January.
The text was signed by the head of the commission under the
supervision of UN and US representatives he said, adding that there was "absolutely no manipulation or changes".
The misunderstandings over the text arise from the fact that delegates were given the text that was printed during the night of 3 January before final changes were made by the assembly, Wardak said.
Mansur says changes are made to the wording in sections that cover official languages, presidential powers and duties and the administration.
Mansur has previously said his bloc will have a candidate ready to contest the presidential elections, scheduled for June.