ST. PAUL, Minn. — Demonstrations near the Republican National Convention site turned violent today, as protesters harassed some delegates, smashed windows, slashed car tires and threw bottles. Police using pepper spray arrested more than 50 people.

The trouble happened not far from the Xcel Energy Center convention site where the GOP was starting its four-day meeting. And many of those involved in the more violent activities identified themselves as anarchists. These protesters, clad in black, were operating on the streets in addition to a mostly peaceful anti-war march, wreaking havoc by damaging property and setting at least one fire. Most of the trouble was in pocket of a neighborhood near downtown, several blocks from where the convention was taking place.

Police estimates of the crowd shifted several times during the event, ranging from 2,000 to 10,000. The crowd was clearly in the thousands. Late this afternoon, long after the antiwar marchers had dispersed, police requested and got 150 Minnesota National Guard soldiers to help control splinter groups near downtown.

Members of the Connecticut delegation said they were attacked by protesters when they got off their bus near the Xcel Center, KMSP-TV reported. Delegate Rob Simmons told the station that a group of protesters came toward his delegation and tried to rip the credentials off their necks and sprayed them with a toxic substance that burned their eyes and stained their clothes.

One 80-year-old member of the delegation had to be treated for injuries, and several other delegates had to rinse their eyes and clothing, the station reported.

Five people were arrested for lighting a trash bin on fire and pushing it into a police car, St. Paul police spokesman Tom Walsh said. Authorities didn’t have immediate details on the other arrests.

The antiwar march was organized by a group called the Coalition to March on the RNC and Stop the War, whose leaders said they hoped for a peaceful, family-friendly event. But police were on high alert after months of preparations by a self-described anarchist group called the RNC Welcoming Committee, which wasn’t among the organizers of the march.

About 20 people dressed in black tried to block a key intersection. Police quickly dispersed the group, then shot two tear gas canisters at them as the fled.

Pictures taken by photographers showed officers using pepper spray on people who appeared to be trying to block streets.

Up to 200 people from a group called Funk the War noisily staged their own march. Wearing black clothes, bandanas and gas masks, some of their members smashed windows of cars and stores. They tipped over newspaper boxes, pulled a big trash bin into the street, bent the rearview mirrors on a bus and flipped heavy stone garbage bins on the sidewalks.

One member of the group carried a yellow flag with the motto “Don’t Tread on Me.” The group chanted: “Whose streets? Our streets!”

At one point, people pushed a trash bin filled with trash and threw garbage in the streets and at cars. They also took down orange detour road signs. One of them used a screwdriver to puncture the back tire of a limousine waiting at an intersection and threw a wooden board at the vehicle, denting its side. Another hurled a glass bottle at a charter bus that had stopped at an intersection. The bottle smashed into pieces but didn’t appear to damage the bus.

After the official march ended, police spent hours dispersing smaller groups of protesters, employing officers on horses, smoke bombs and tear gas.

Protesters put eye drops in each other’s eyes after police used chemical irritants such as pepper spray and tear gas. Some wore bandanas and masks to protect themselves.

Protesters were seen lying on an interstate exit ramp to block traffic in downtown St. Paul and linking arms to block other roads.

Terry Butts, a former Alabama Supreme Court justice who is a convention delegate, was on a bus taking delegates to the arena when a brick through the window sprayed glass on him and two others. Butts said he wasn’t hurt.

“It just left us a little shaken,” he said. “It was sort of a frightening moment because it could have been a bomb or a Molotov cocktail.”