Felipe Calderon has been sworn in as Mexico's president to jeers, after brawls in Congress between lawmakers, divided by the nation's tight election.
Felipe Calderon called for dialogue after a stormy inauguration
Just an hour before the ceremony, deputies seized the speaker's platform and blocked the doors of the chamber.
Members of the left-wing Democratic Revolution Party say Mr Calderon, a conservative, won July's poll by fraud.
The row followed an unusual midnight ceremony in which the outgoing president handed power to Mr Calderon.
Opposition deputies argued Mr Calderon could not become president without taking the oath of office - as outlined in the constitution - and tried to derail Friday's official inauguration.
Mr Calderon bypassed the barricaded doors and, surrounded by his supporters and flanked by outgoing President Vicente Fox, he rushed through the presidential oath.
The national anthem was then played over the din of insults made by dozens of opposing lawmakers.
In his inauguration speech Mr Calderon said: "I will always be willing for dialogue, but I won't wait for dialogue before starting work."
After the inauguration opposition leader Andres Manuel Lopez Obrador led tens of thousands of supporters through the centre of Mexico City and vowed to "defend democracy in our country".
Left-wing deputies accuse the conservatives of winning by fraud
In the six months since the election, Mexico has witnessed a succession of protests against Mr Calderon over claims the poll was rigged - something he and the courts have rejected.
Mr Lopez Obrador lost the run-off by half a percentage point and last week launched a "parallel government" in a mock swearing-in ceremony.
BBC Americas Editor Will Grant says Mr Lopez Obrador appears intent on upsetting Mr Calderon's presidency at every turn.
Mr Lopez Obrador has a strong support base, particularly in Mexico City where he was mayor for several years. His party remains in power there and he won a strong showing in the national Congress.
Mr Calderon has tried to focus on the social and economic challenges but, no matter how worthwhile his agenda, if the inauguration was anything to go by, he may be facing a difficult future, our correspondent says.