The opposition has repeatedly warned that failure to act will further inflame mass anti-government protests.A protest rally is also expected outside the parliament building in the capital Kiev.Pro-presidential MPs - who have a majority - have opposed attempts to return to the 2004 constitution.In a separate development, Russian Finance Minister Anton Siluanov said Moscow would provide Ukraine with a new $2bn (£1.2bn) tranche of aid "this week".In December, Moscow pledged $15bn to back Ukraine's struggling economy, but so far only $3bn has been transferred.The Kremlin had hinted it would freeze the aid until a new government acceptable to Moscow was formed after Ukraine's PM Mykola Azarov resigned last month.Ukraine's unrest began in November, when Mr Yanukovych backed away from a landmark association and free trade deal with the European Union, and instead agreed the loan with Russia.Moscow wants Ukraine to join the Russia-led Customs Union, where Belarus and Kazakhstan are also members.Both the EU and Russia have accused each other of interfering into Ukraine's affairs.Snap election demandUkraine's lawmakers are due to begin their parliamentary session at 10:00 local time (08:00 GMT).The opposition has for weeks been pushing for a return to the 2004 constitution, which would mean President Yanukovych losing some of the powers he has gained since his election in 2010.The changes would mean that parliament - not the president - would be appointing the prime minister and most cabinet members as well as regional governors.The move could also lead to snap presidential elections - a key demand by the opposition.Pro-presidential lawmakers have argued that different proposals on how to return to the previous constitution in a legitimate way have not been thoroughly discussed and more time is needed to iron out all the differences between the two sides.Meanwhile, anti-government protesters are planning to march to the parliament building to demand the constitutional changes.The parliament and key government buildings have been guarded by riot police and interior troops, and there are fears that clashes may erupt later on Tuesday.Amnesty dealOn Monday, an amnesty for anti-government protesters in Ukraine came into force after protesters had ended their occupation of government buildings in Kiev and in the regions.The prosecutor's website said criminal charges would be dropped after the opposition had left Kiev city hall and other locations.Protesters had held some of the buildings for more than two months.But a sprawling tent city - widely known as Maidan - remains in Kiev's central square, where some denounced the decision to end the occupations.The protesters also partially dismantled barricades on Hrushevskyy Street, which was the scene of deadly clashes earlier this year.On Friday, authorities freed the last of 243 prisoners who were arrested during the unrest.The opposition is now pressing for all pending criminal cases to be closed - the authorities say this will be done within a month.Lawmakers passed the amnesty law last month and agreed to negotiate with the opposition after at least four people were killed in protests.(BBC)ANN.Az