Opposition lawmakers in Zambia walked out of parliament Wednesday, saying President Michael Sata and his government had been violating the constitution.

"As members of parliament, we pledged to uphold the constitution of the Republic of Zambia and we feel that this government has been breaching the constitution," parliamentary opposition leader Felix Mutati told journalists.

"They have been creating ministries and transferring some responsibilities from other ministries to another."

The walk-out came two days before UN Secretary General Ban Ki-moon is due to address a special session of parliament, raising fears the opposition could again head for the exits during his speech.

Last week, Sata created a new ministry of defence in charge of rural development and national service, and appointed a lawmaker from his Patriotic Front (PF) party as deputy minister.

Sata took office in September after defeating incumbent Rupiah Banda, whose Movement for Multiparty Democracy (MMD) had been in power for 20 years.

The MMD lost its parliamentary majority in the same election.