Bronislaw Komorowski's inauguration before Poland's parliament came after a moment of silence for Lech Kaczynski and the 95 other people killed in the crash near Smolensk, Russia, on April 10.
"Eternal rest, grant unto them, O Lord," intoned a lawmaker from the late leader's conservative party, Ryszard Bender, before the assembly fell silent.
Several lawmakers were killed in the crash. Their seats were still empty during the ceremony, with photographs of them and flowers in their places.
In the first speech of his five-year term, Komorowski pledged to work to help modernize Poland, promoting scientific research and working to improve the creaky state health system. The pro-European Union leader also said his first official trips abroad would be to Brussels, Paris and Berlin.
He also pledged to support an ongoing rapprochement with Russia, the huge eastern neighbor with which post-communist Poland's relations have sometimes been tense.
"There will be no stable development in our region without the cooperation of Russia," Komorowski said.
He said that, despite the grief the plane crash unleashed, it also revealed the resilience of the young democracy only 20 years after it threw off communist rule.
"Smolensk was our common tragedy and our common mourning," he told a gathering that included both houses of parliament, Prime Minister Donald Tusk and his two surviving democratically elected predecessors, Lech Walesa and Aleksander Kwasniewski.
"It also showed us all that our society, constitution and democracy, can rise to such a situation," Komorowski said. "The order which we have built over the last 20 years in Poland managed to maintain the continuity of power and to honor the memory of the victims with dignity."
"It is my duty to remember those who died at Smolensk," he said.
Komorowski, 58, takes on a role with symbolic weight but limited real powers, though he will have the power to veto laws and have a say over the country's military missions abroad.
The most important of those missions is that in Afghanistan, which Komorowski has said he hopes to end in 2012.
He is a leading member of the governing party of Prime Minister Tusk, Civic Platform, which favors pro-business policies including further privatizations and harmonious relations with Brussels and EU neighbors such as Germany.
Many political observers hope that having a president and government from the same party will usher in domestic political calm and end bitterness between the government and president that marked the three years before Lech Kaczynski's death.
Komorowski beat Kaczynski's identical twin brother Jaroslaw Kaczynski, leader of the conservative Law and Justice party, in the presidential election last month.
