(Updates with Gambari in Malaysia, para 11)

By Ed Cropley

BANGKOK, Oct 17 (Reuters) - Dissidents on the run in military-ruled Myanmar called on the United Nations Security Council on Wednesday to impose a blanket arms and investment ban on the junta to try to force it towards democratic reform.

In a letter written from hiding to U.N. Secretary-General Ban Ki-Moon, three members of the "88 Generation Students Group" said the generals were duping the international community into thinking they were serious about relaxing their grip.

"This may be the last letter we send to you before our own arrest and torture and we send it with the utmost urgency," the trio -- Tun Myint Aung, Nilar Thein and Soe Htun -- wrote in the letter, a copy of which was obtained by Reuters in Bangkok.

The "88 Generation" was a group of student activists who led a major anti-junta uprising in 1988 eventually crushed by the army with the loss of an estimated 3,000 lives.

They were also behind the small fuel price protests that snowballed into last month's monk-led demonstrations against 45 years of military rule in the former Burma. At least 10 people were killed when soldiers were sent in to end the protests.

Nearly all the leading lights in the group, most of whom spent years behind bars after the 1988 uprising was crushed, have already been arrested in the latest crackdown, still going on despite the international clamour for it to end.

The official New Light of Myanmar newspaper said 2,927 people had been detained across the country, but only 468 remained in custody, a figure opposition, human rights and exile groups suspect is probably too low.

The National League for Democracy (NLD), the opposition party that won a 1990 election landslide under the leadership of Aung San Suu Kyi only to be denied power by the army, says more than 200 of its members have been rounded up.

It is not known how many, if any, have been released. On Tuesday, five NLD members involved in protests in the northwestern state of Rakhine were jailed for up to 7-½ years after closed trials, relatives said.

Buddhist monks freed after more than a week of interrogation have told Reuters of animal-like conditions in makeshift detention centres without toilets or drinking water. Monks were also kicked and beaten during questioning, they said.

RAIDS CONTINUE

United Nations special envoy Ibrahim Gambari, now in Malaysia on an Asian tour to build a common front to persuade the generals to compromise with Suu Kyi, who has spent 12 of the past 18 years in detention, appealed on Monday for the arrests to stop.

Despite his plea and quiet on the streets, the nightly raids have continued, and state media say there will be no opposition to the junta's seven-stage "roadmap to democracy", which critics say will simply cement the army's iron grip on the country.

"We will go ahead. We will not deviate from our path," the New Light said in a defiant commentary.

"We will get rid of the barriers and obstacles on the way."

Senior General Than Shwe has agreed to meet Suu Kyi, whom he is widely known to loathe, although his preconditions -- that she abandon her "confrontation" and support for sanctions and "utter devastation" -- cast doubts on the sincerity of his offer.

The NLD welcomed the appointment of a big-hitter within the junta as a go-between for 62-year-old Suu Kyi and Than Shwe, 74, but said any talks must have no prior strings attached.

In their letter to Ban, the '88 students also called on China and Russia -- both major arms suppliers to the junta -- not to wield their veto against any action by the Security Council, as they did in January.

Despite a rare call for restraint, Beijing has made it clear it will not allow concrete steps against Myanmar, home to huge reserves of natural gas coveted by energy-hungry neighbours.

The European Union agreed on Monday to tighten existing sanctions to target trade in gems, timber and precious metals -- all major junta money-spinners.

Tokyo said it would halt 550 million yen ($4.7 million) in aid in protest at a Japanese journalist being shot dead by soldiers clearing the streets of Yangon of protests on Sept. 27.

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