TOKYO (AP) – Japan for the first time Sunday officially marked the 61st anniversary of the day it regained sovereignty following its World War II defeat, as Prime Minister Shinzo Abe steps up his nationalist campaign.

After taking over as prime minister in December, Abe focused mainly on improving Japan's slumping economy. But he recently has shifted his focus to pursuing his conservative agenda, and the marking of the anniversary is seen as a step to drum up support for revising Japan's U.S.-inspired pacifist constitution. Abe's conservative party has for years denounced the current constitution as one imposed by the United States, which occupied Japan from the end of World War II until 1952.

Last month, the Cabinet approved a plan by the ruling party to designate April 28 as Japan's ``sovereignty recovery'' day, and Sunday's ceremony was the first government-sponsored event to mark the day. Similar events were previously held privately among ultra-conservative lawmakers, mainly from Abe's Liberal Democratic Party, and their right-wing supporters.

Sunday's proceedings were filled with nationalistic rituals considered symbols of the imperial warship that drove Japan to its 20th century aggression in Asia. The ceremony started with the singing of the controversial national anthem ``Kimigayo'' ("His Majesty's Reign''), and ended with ``Banzai!'' cheers for Emperor Akihito. There also was a huge rising sun decoration on the center stage.

During the ceremony, Abe urged Japanese to mark the day in their hearts, pledging to make Japan a stronger country with national pride. He said that 61 years ago, Japanese had high hopes and commitment to make a better Japan, adding that people today must live up to the expectations.

``We are obliged to make Japan strong and tough so our country becomes one that the rest of the world can count on,'' Abe said. He said he was seeking to make Japan a better and more ``beautiful nation,'' a favorite phrase of his, but one critics say has a nationalistic undertone.

The ceremony was the latest in a series of nationalistic events and remarks that have invited harsh reactions from neighboring countries that suffered from Japan's wartime aggression.

Visits by several government ministers and nearly 170 lawmakers to Tokyo's war shrine this month have enraged China and South Korea. Japanese political leaders' visits to the Yasukuni Shrine _ which memorializes 2.3 million war dead, including 14 wartime leaders convicted of war crimes _ have been a constant point of contention with those countries. 

Abe also infuriated China and South Korea by saying that there is no clear definition of ``aggression'' and that Japan will not ``succumb to any threat.'' 

Abe has also campaigned for recognizing Japan's Self-Defense Forces as a full-fledged national military, for revising Japan's past apologies for atrocities committed by its Imperial Army before and during World War II, and for upgrading the emperor's status to head of state, as outlined in the Liberal Democratic Party platform.

Celebrating April 28 as sovereignty recovery day invited harsh criticism from the southern island of Okinawa, where U.S. occupation continued until 1972. In the Okinawan city of Ginowan, thousands of protesters staged a rally in a park to protest the ceremony.

Okinawa is currently home to nearly three-quarters of the U.S. troops stationed in Japan under a bilateral security pact, causing deep-rooted anti-U.S. sentiment on the island, as well as distrust toward Japan's central government for not doing enough to relieve the burden. Okinawan Gov. Hirokazu Nakaima boycotted Sunday's ceremony.