* PM's ruling bloc takes solid majority - media

* Ruling bloc with allies falls short of 2/3 of upper house

* PM Abe said to seek constitution revision during term

* Turnout below 50%, second-lowest in post-war election (Updates with the number of votes, paragraph 4)

By Linda Sieg and Kiyoshi Takenaka

TOKYO, July 21 (Reuters) - Japanese Prime Minister Shinzo Abe's ruling bloc won a solid majority in an upper house election on Sunday but his coalition and allies fell short of a two-thirds majority needed to begin revising the pacifist constitution, public broadcaster NHK said.

Abe, who took office in December 2012 pledging to restart the economy and bolster defence, is on track to become Japan's longest-serving premier if he stays in office until November, a stunning comeback after he abruptly ended a first, troubled one-year term in 2007.

However, turnout fell below 50% for the first time in a national election since 1995, a sign many voters feel they lack an attractive option. It would be the second-lowest level since records began after World War Two, the government said.

Abe's Liberal Democratic Party (LDP) and its junior partner, the Komeito party, were assured 71 of the 124 seats being contested in parliament's 245-seat upper house, NHK showed.

However, NHK said the ruling bloc and its allies fell short of the 85 seats needed to retain the two-thirds "super majority" required to begin revising the constitution's pacifist Article 9 to further legitimise the military, a controversial step.

Abe said the size of the victory showed voters wanted to debate changing the charter for what would be the first time since its enactment after Japan's defeat in World War Two.

"Of course, we cannot take the timing as a given, but I would like to achieve it (constitutional reform) somehow during my term," Abe said on television on Sunday evening. His term as LDP president runs until September 2021.

Changing the charter would be hugely symbolic, underscoring a shift away from post-war pacifism already underway.

Article 9, if taken literally, bans maintenance of a military but has been stretched to allow armed forces for self-defence.

Without the two-thirds majority, Abe is likely to try to lure other opposition lawmakers to back his proposal to enshrine the military in the constitution, but that could be tough with a lower house election certain between now and 2021.

VOTERS DIVIDED

Surveys show voters are divided over changing the charter, with opponents worried doing so would increase the risk of Japan getting entangled in U.S.-led conflicts.