Tymoshenko, second in opinion polls ahead of the election in January, vowed in a two-hour speech to 5,000 supporters of her Batkivshchyna (Homeland) party she would proceed quickly with proposals for constitutional change.
"The first thing we will do is to conduct a nation-wide referendum...with a single question -- what form of government the people of Ukraine want," she said, dressed in her party's white colours and sporting her trademark peasant braid.
"The answer is very simple -- a presidential system with no prime minister...or a parliamentary system with only a prime minister elected by the people...But it is not for politicians to decide this, but rather the people in a referendum."
Tymoshenko said she had no intention of dissolving Ukraine's fractious parliament, where heated debate frequently degenerates into fistfights or blockades halting its activity. She pledged instead to "unite this parliament and strengthen it".
The prime minister has been locked for months in debilitating policy rows with President Viktor Yushchenko, her estranged ally from the 2004 "Orange Revolution".
Ukraine's last major constitutional change -- cuts in the president's powers -- were approved at the height of the 2004 protests against poll fraud that vaulted Yushchenko to power.
With the economy shrinking over 20 percent in the first quarter of 2009, nearly all politicians have been calling for some form of constitutional change to halt the rows and improve the functioning of institutions.
Twice prime minister since 2004, Tymoshenko is credited with about 15 percent support behind former premier Viktor Yanukovich, with about 20 percent.
Lying in third place is Arseniy Yatsenyuk, who portrays himself as Ukraine's "new face", unlinked to the various rows.
Yanukovich's political base is in Russian-speaking eastern industrial regions, while Tymoshenko draws much of her support from the nationalist west and central areas. Yatsenyuk said he hopes to overcome the country's traditional divisions.
Yushchenko is running for re-election but is widely derided by ordinary Ukrainians and his rating stands in single figures.
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