With 91 percent of the votes counted from yesterday's runoff, Kaczynski led rival Donald Tusk of the pro-market Civic Platform party, 55.5 percent to 44.5 percent, according to the State Election Commission.
The 56-year-old Kaczynski's victory clears the way for the start of coalition talks today between the two right-of-centre parties which are allies, and have said they intend to form a government together after winning a combined majority in the 25 September parliamentary election.
During the campaign, Kaczynski vowed to root out former communists and fight political corruption, but sounded a conciliatory note as he claimed victory late yesterday.
He urged Tusk's Civic Platform to finish work on a coalition government with Law and Justice, which is headed by his twin brother, Jaroslaw Kaczynski.
"Poland needs wrongs to be accounted for, but even more Poland needs accord. I want to reinstate that accord," Kaczynski said. "I want to now address my friends in Platform to ask them to quickly complete work on a new government."
Tusk conceded defeat. "Today I must tell myself I did not make it," he told glum supporters at his election headquarters.
Tusk led the first round of presidential voting two weeks ago with 36 percent of the vote, with Kaczynski placing second with 33 percent. The runoff between the top two candidates was held because none of the 12 candidates received more than 50 percent of the vote.
In the runoff campaign, Kaczynski forged ahead as voters responded to his warnings that free-market policies must not cut social welfare for the less fortunate.
Tusk won in the most prosperous, western regions of the country, while Kaczynski swept the poorer east, exit polls showed.
The mild-mannered Tusk made some wonder whether he was tough enough to be president, in contrast with the aggressive Kaczynski, a populist who tried to stop a gay rights parade and issued Germany a bill for damage done during the World War II occupation.
In the last week of the campaign, Kaczynski won a key endorsement from anti-European Union populist Andrzej Lepper of the left-wing Self-Defense party.
Lepper received only 15 percent of the vote in the first round, but surveys showed that more than 80 percent of his supporters' votes went to Kaczynski.
Kaczynski's promises to stand up to Germany - even though the two countries enjoy good relations - appeared aimed at older voters who remember the war. His promises to keep pensions and social benefits apparently helped him win voters over 60 by a 61-39 percent margin, exit polls for TVN24 showed.
"He thinks about poor people, about retired people and children, and we are retired, that's why we voted for him," said Danuta Niemkowska, a 71-year-old retired teacher, after she and her husband voted at a school in Warsaw's riverside district.
Both candidates are right of center, but Tusk is more oriented toward market economics and favors a flat tax. Kaczynski supports tax cuts, but prefers the system under which high earners pay more and advocates tax breaks for those with large families. His campaign also stressed Roman Catholic stands such as opposition to abortion and gay rights.
The two Kaczynski brothers, both former activists in the Solidarity free trade union movement which ended communist rule, won fame as child stars in a hit film, "Two Who Stole The Moon." But their resemblance became a political handicap, pushing Jaroslaw to abandon his claim to become prime minister in favor of a little known party official Kazimierz Marcinkiewicz after last month's parliamentary election.
Lech Kaczynski said he would leave Law and Justice; although there is no requirement that he do so, the president is regarded as above day to day politics, and outgoing President Aleksander Kwasniewski quit his party after being elected.
Kwasniewski, a former communist popular for his easy style, has served his maximum of two five-year terms and could not run again.
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