(Adds lawmakers' statement in paragraphs 10-11)

By Chisa Fujioka

TOKYO, April 27 (Reuters) - Japan hanged three convicts on Friday, the Justice Ministry said, the first series of executions after the number of death row inmates rose earlier this year to more than 100 -- almost twice the figure of a decade ago.

As usual, the ministry did not identify the three, but media said those executed included Yoshikatsu Oda, who was convicted of murdering two people for insurance money in 1990.

The executions were rare in that they took place while parliament is in session, although the timing is still just ahead of a series of national holidays next week. Prime Minister Shinzo Abe was away on a high-profile visit to the United States.

Japan carries out executions several times a year, usually when parliament is in recess or in December when the country is winding down for the New Year holidays, in what critics say is a strategy to avoid discussions of capital punishment.

Executions last took place on Christmas Day 2006, when four convicts were hanged.

In Japan, condemned inmates and their relatives are not told of an impending execution until the day it takes place, a practice many say is inhumane.

Amnesty International Japan issued a statement calling for Japan to abolish the death penalty, saying most countries have already done so.

"We strongly protest against today's executions," it said.

"Despite the Justice Ministry saying it would be cautious in carrying out executions, those today came quickly, just four months after the last in December," it added.

Lawmakers opposed to the death penalty wrote to Justice Minister Jinen Nagase, saying the executions were "problematic".

"We call on Minister Nagase not to think simply that carrying out executions is his duty, but to acknowledge that questioning the issue of capital punishment is also a part of his job," the group, led by lawmaker Shizuka Kamei, said in their letter.

Capital punishment is little questioned by most Japanese, who polls say support the death penalty in the face of a recent rise in violent crime.

The number of inmates on death row in Japan hit 100 in February, and experts say intense media coverage of violent crimes and a vocal victims' rights movement have led Japanese courts to hand down stiffer penalties, including more death sentences.

The mood in Japan contrasts with that in the United States, which along with Japan is one of the few wealthy democracies to execute criminals.

Death sentences in the United States fell to a 30-year low in 2006 and capital punishment is now under what appears to be an unprecedented review amid eroding support.

Seiken Sugiura, Nagase's predecessor as Justice Minister, caused a media fuss by saying on his appointment in 2005 that he would not sign death warrants.

He later withdrew the remarks, but no executions took place while he was in office.

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