The last 12 hours of his 51-day hunger strike were the hardest, activist Mitch Snyder recalled yesterday.

Bedridden, in pain from severe dehydration and at times lapsing into unconsciousness, Snyder on Sunday had concluded hours of often-heated telephone negotiations with Health and Human Services Secretary Margaret Heckler. Heckler, who was calling from the White House, had agreed to most of the renovations Snyder sought for a downtown Washington shelter for the homeless, but the president had not approved the agreement and there was nothing in writing.

"I was getting hopeful, but I was also worried it wouldn't work out, that something would go wrong," Snyder said yesterday, barely 24 hours after ending his fast. "The hardest time is when things begin to happen. . . . I wondered if I was going to die before seeing it work out," he said, as he lay in his bed in the intensive care unit at Howard University Hospital, an intravenous tube pouring nutrients into his arm, his heart monitored by a computer.

The 41-year-old Snyder not only lived to learn that the president had agreed to most of his demands to turn the decaying 800-bed shelter into a model facility, but felt well enough yesterday to hold a press conference.

Wearing a red-plaid bathrobe, Snyder, flanked by his doctors and members of the Community for Creative Non-Violence which he leads, was rolled into a conference room in a wheelchair to face a crowd of reporters from around the world. During a 45-minute press conference he thanked God for sparing his life and President Reagan "who took time out to deal with this." Reagan agreed to Snyder's demands Sunday aboard Air Force One while en route to a campaign stop.

Snyder, who had pledged to starve himself to death unless the administration responded to his demands, said his hunger strike, begun Sept. 15, was strategically timed for both the expected advent of cold weather -- "who knew it would be summer forever?" -- and today's presidential election.

"We wanted to put the question of homelessness on the national agenda," said Snyder, who added, "If the polls are anywhere close to right the president doesn't need us."

Doctors said yesterday they expect Snyder, whose condition was upgraded from serious to fair, will be released tomorrow. They do not know whether he has suffered permanent eye damage, as he did two years ago when his 64-day fast helped persuade President Reagan to change the name of the nuclear submarine from the Corpus Christi to the City of Corpus Christi.

Snyder, who lost 62 pounds, probably will not be allowed solid foods for several days, a fact he finds enormously frustrating. "If left to my own devices, I would probably eat until my stomach burst," he said. "I am just ravenous, especially psychologically, which is the worst kind of hunger."

Since Snyder ended his fast, CCNV's communal house at 1345 Euclid St. NW has been swamped with telephone calls. Among them was one from a Hollywood producer interested in making a movie about him and several anonymous threats to bomb CCNV's 800-bed shelter at 425 Second St. NW.

As Snyder was meeting with the press, U.S. officials led by Harvey Vieth, the chairman of a federal task force on homelessness, were meeting to discuss how they would turn the squalid, vermin-infested facility, one of the largest in the nation, into the "model physical-shelter structure" Heckler had promised.

Vieth said an exterminator had been dispatched to the shelter and that plumbers were trying to repair the sprinkler system and bathrooms.

Currently, according to the shelter staff, only one shower, six sinks and 10 toilets -- shared by 700 homeless men and CCNV members -- are working.

Heckler also pledged to construct laundry and kitchen facilities, neither of which now exist. Since the shelter opened 10 months ago, CCNV members have cooked soup in huge vats in the kitchen of CCNV's communal house and taken it by van to the shelter.