"I don't want war. I don't want revenge," Petro Poroshenko said after taking the oath of office.
But then his talk turned tough.
He promised to meet anyone challenging Ukraine's territorial integrity with military might.
Alluding to a Biblical verse he said, "Who comes with the sword will fall from the sword."
The statement was met with enthusiastic applause.
The country will build the means to do so, Poroshenko said, and re-arming the Ukrainian army must be a priority.
"No one will protect us, if we do not learn to protect ourselves."
He called for separatists in Ukraine's east who have taken up arms against the government to lay them down and offered amnesty to those who "do not have blood on their hands."
Poroshenko also called for corridors to open to allow fighters who have joined pro-Russia separatist forces from outside the country to leave Ukraine.
In Russia, President Vladimir Putin ordered border guards to stop the illegal crossings of people from Ukraine by beefing up security, the state-run ITAR-Tass news agency reported.
Meanwhile, in Donetsk on Saturday, an assistant to Denis Pushilin, the head of the self-proclaimed Donetsk Republic, was killed in an attack in broad daylight, a CNN crew at the scene said.
Attackers shot at the car that Maksim Petruhin was in, and when he exited the vehicle, he was shot and killed.
It was the most brazen attack in Donetsk since the unrest began.
Poroshenko, one of the world's richest men and an experienced politician, took the oath of office in the country's parliament Saturday.
He was presented with the symbols of high office -- a presidential badge, a seal and a mace -- before taking to the podium for his inaugural address.
His opening remarks left little doubt over the tenor of his speech, as he praised the activists of the Maidan, whose rebellion in the center of Kiev led to the ouster of his predecessor in office, Viktor Yanukovych.
The pro-Russia former President sparked rage from Ukrainians who wanted the country to join the European Union when he nixed an agreement with the EU in favor of closer ties to Moscow.
Ensuing street battles resulted in the deaths of dozens.
Poroshenko condemned the former pro-Russia government as a "dictatorship."
"The people stood up," he said, declaring his country's new direction toward the West.
Before at least four European Presidents and U.S. Vice President Joe Biden, the man who became a billionaire as a chocolate maker vowed to quickly continue Ukraine's move toward the EU, calling Ukraine's path to Europe "irreversible."
Ukraine's natural place is with Europe, he said, adding that he's prepared to sign an economic partnership agreement with the European Union as soon as its officials approve it.
The agreement is just one step toward Ukraine becoming a member of the European Union.
Poroshenko received a standing ovation for the remark.
Ukrainians have reason to look forward to a brighter future, he said.
"We are being supported by the whole world."
But the new President inherits a country with a burgeoning armed conflict in its east.
Violence there is taking lives daily, and government troops and rebel fighters are beefing up their might.
The names of the cities Donetsk and Luhansk have transformed into monikers for flashpoints of tension between Russia and the West as the government in Kiev breaks its ties with Moscow to embrace Europe and the United States.
But ethnic Russian separatists in the east and south yearn to stay with Moscow.
The Kremlin, holding fast to centuries-old ties, is said to be helping them dig in their heels after annexing the region of Crimea, which before 1954 was part of Russia.
Poroshenko addressed the move directly.
The territorial integrity of Ukraine is not up for discussion, he said, and he has taken an oath to uphold it.
"I will stand by this oath no matter what," he said.
He promised to visit Luhansk and Donetsk and maintain respect for the Russian language, commonly spoken in Ukraine's east and south.
The government will be decentralized, giving those regions more say in their own affairs, he said.
"New powers will be allocated to regional powers," Poroshenko said.
But there will be no federalization, as many separatists have demanded.
Some analysts believe such a move would help cement Russia's influence in their regions.
Ukraine will be one unified country, the President said.
Security prospects have seemed elusive, but on Friday, a ray of hope pierced the ominous clouds between Moscow and Kiev during D-Day celebrations in France, where Putin came together with Western leaders to honor common sacrifices made to defeat fascism in World War II.
Putin came face-to-face with Poroshenko.
Afterward, Poroshenko announced that negotiations between the two sides would begin Sunday, his second day in office.
But in his address on Saturday, he spoke of the meeting with Putin in more assertive language.
"Yesterday I made a firm statement about it to the Russian leadership in Normandy," he said.
"Crimea was, is and will be Ukrainian soil."
Source: CNN News Europe