Robert Stolarik for The New York Times

TAMPA, Fla. — The storm that caused Republican officials to cancel the bulk of the first day of their convention is also dampening the protests, as fewer demonstrators than were expected turned out for the first day of rallies.

A morning gathering that organizers had said they thought would include at least 5,000 people drew only a couple hundred people who rallied and marched in a drizzle that sometimes yielded to baking heat.

And later in the afternoon, a tense standoff between protesters and the riot police ended as a heavy downpour sent some protesters racing for cover under awnings, while others danced in the rain.

The rain was a looming presence even when it was not falling, as one of the organizers of a morning march and rally acknowledged from a stage at Perry Harvey Sr. Park.

“We all know the affect the weather has had on the turnout of this rally,” said the organizer, Mick Kelly, adding, “It’s a lot lower.”

The small crowds that took to the streets on Monday were in contrast to the thousands who marched during the opening days of previous conventions in St. Paul, New York and Philadelphia. But the events in Tampa also included less friction between the police and protesters. During the two marches on Monday, it appeared that only one man was arrested.

Just before noon a few hundred people left Perry Harvey Sr. Park and embarked on what organizers said was a permitted march, chanting, blowing horns and waving signs criticizing Republican stances on various issues as they headed toward a designated protest area near that the Tampa authorities had set up in a lot near the convention center.

Among those who watched the crowd was the Tampa police chief, Jane Castor, who told reporters that everything appeared to be going well.

A few minutes later, as the marchers streamed into the large fenced lot that had been set aside for protesters, some began loudly shouting that the protest area was a “cage.” Soon, several dozen younger people, some dressed in black or tying bandannas over their faces left the area, chanting: “Whose streets? Our streets. Tear up the concrete.”

The police followed them, and after several blocks one of the marchers was arrested in a parking lot near the intersection of Scott Street and Morgan Street. A police chief said that the man had been wearing a mask, something that was forbidden by a special ordinance adopted in preparation for the convention.

“They asked him to take it off and he resisted,” said the chief, J.A. Bennett.

One of the man’s friends said that he had been arrested without being given a chance to remove a bandanna tied over his face.

“The cop tackled him,” said the friend, Keith Cutter, adding that his friend had not struggled.

Many of the protesters reassembled on Monday afternoon at the encampment that has come to be known as Romneyville near the edge of downtown Tampa. They then set off on a permitless march organized by the Poor People’s Economics Human Rights Campaign that was meant to address the problems caused by poverty.

One of the marchers, Jennifer Sullivan, 58, a mail carrier from Spring Hill, Fla., said that both major political parties were delinquent in addressing many issues that affect the poorest Americans.

“They both talk about the middle class,” Ms. Sullivan said. “But they both go to K Street to the lobbyists.”

After several blocks, most of the marchers left the street and entered Lykes Gaslight Square Park on Kennedy Boulevard. But several dozen remained in the street and faced off against a line of officers who blocked an intersection on Kennedy Boulevard.

After several minutes those protesters moved onto sidewalks, and large detachments of officers equipped with plastic shields and a few rifles with orange stocks labeled “less lethal” arrived and stood in ranks.

As protesters scoffed from sidewalks and accused the police of overreacting, heavy rain poured from the sky. Most marchers took cover in doorways and under building overhangs, but some, followed by police officers on bicycles, marched down the center of North Tampa Street until they got back to their encampment, where they sought shelter under tents and tarpaulins.