President Vladimir Putin (search) signed a bill Sunday to end the election of governors by popular vote, while more than 1,000 opposition activists converged on Constitution Day to denounce what they called his increasingly authoritarian rule.

Putin notably chose Sunday to sign one of two measures that critics say could violate Russia's constitution, which was adopted in 1993 under his predecessor, Boris Yeltsin (search), and was considered one of the main democratic achievements of Yeltsin's rule.

The Federation Council (search), or upper house of parliament, gave the measure the final stamp of legislative approval on Wednesday with a vote of 145-1.

Putin on Sunday also denied he seeks to change the constitution — a concern raised by critics who fear his administration might seek amendments to keep him in power past 2008. The constititution allows only two consecutive four-year terms.

The new law gives the president the right to appoint governors, who would then be confirmed by regional legislatures. Russia consists of 89 regions, whose leaders are currently chosen by popular election.

Under the law, if lawmakers reject the president's candidate twice, he could make a new nomination, appoint an acting governor or dissolve the legislature. If a candidate is rejected for the third time, the president can dissolve the legislature without waiting for consultations to play out.

The other main proposal, which would end the election of direct election of national lawmakers, is also expected to receive swift parliamentary approval.

Putin proposed the changes in the wake of terrorist attacks last summer that killed more than 450 people, saying the reforms would enable better security. However, the changes have alarmed opposition parties and human rights monitors.

On Sunday, more than 1,000 activists, politicians and prominent Kremlin critics gathered at the opening of the All-Russian Civil Congress for Democracy and Against Dictatorship. The meeting made for an unusual alliance of liberals and communists.

"These are all very different people, but we have been united by one common concern — the authorities' outright encroachment on our rights," said respected rights activist Lyudmila Alexeyeva.

"If we don't show solidarity in defending our rights, we will lose every single one of them," said Alexeyeva, head of the Moscow Helsinki Group.

Yevgeny Kiselyov, a prominent journalist, criticized a bill that would scrap the Constitution Day holiday starting next year. He said the bill, to which lawmakers gave preliminary approval last month, was a Kremlin effort to downplay the importance of the constitution in order to make changing it easier.

Critics have expressed concern that the Kremlin, making use of the loyal parliament, might initiate a constitutional amendment enabling Putin to stay in power after his second and final term in office ends in 2008.

"The choice is simple: in a couple of years, we will be left with either this constitution or these authorities," said Garry Kasparov, a world chess champion and a member of liberal opposition group. "I choose the constitution."

Meeting with Constitutional Court justices on Sunday, Putin said the Kremlin has no plans to alter the constitution. He noted that the court has the power to interpret articles of the constitution.

"This does not at all mean that we are getting ready to somehow change the constitution or make corrections. Such a task does not stand before us; we have no such plans," he said in televised comments.

However, he did not rule out changes.