Members of a truck convoy carrying aid to Nicaragua were turned away at the Mexican border today by U.S. Customs officials and then formed a human chain to block traffic on the principal bridge over the Rio Grande. A Mexican federal officical broke up the bridge blockade about 12:15 p.m. The blockade began when Customs officials detained a convoy truck and its driver as he attempted to cross the bridge in defiance of federal authorities. Earlier Wednesday, Customs agents stopped the Veterans' Peace Convoy to Nicaragua a block from Bridge No. 1 and attempted to route it to an inspection area other than the one at the bridge, but the group refused to budge. ``We're insisting that we be allowed to proceed right up to this inspection area, as previously agreed upon,'' said Gerry Condon, a convoy organizer. ``The difference is we know this inspection area; that's where the media is, that's where our supporters are, that's the agreement we had with them. We're not going to let them lead us by the nose and take us out of sight of people.'' One convoy vehicle, driven by Bob Livesey of Dorchester, Mass., later drove up to the bridge, where Customs officials refused to allow it to turn south toward Mexico. Another vehicle, driven by Hal Muskat of San Francisco, was detained about noon. ``My instructions are that I am not to allow you to leave,'' said James Purser, chief Customs inspector in Laredo. ``We're going to keep on trying to get our vehicles down here to the border one at a time,'' Condon said. As police and Customs officials stood by the roadblock, convoy members and supporters sang the song, ``Give Peace a Chance,'' and chanted ``Let the Convoy Pass.'' Witnesses said a small caravan left the convoy campsite for the bridge five miles away shortly before 9 a.m. About 20 supporters of the group held signs that said, ``Embargo South Africa, Not Nicaragua'' and ``Let The Convoy Pass.'' The group must submit to a Customs inspection to enter Mexico. Customs officials said that the group is being asked to comply with normal export procedures and that it has played up the issue to attract media attention. About 200 convoy members and supporters held a rally Tuesday night at its camp. Many shouted, ``Viva Nicaragua libre!'' (``Long live free Nicaragua!'') when candles were lit during the rally, which featured leftist folk songs and an appearance by Rep. Mickey Leland, D-Texas. ``I'm here to do what I can to see to it that the convoy gets across the border and to Nicaragua to deliver the humanitarian aid that they have,'' Leland said. Notables including Jesse Jackson and actor Ed Asner also have urged the U.S. government to allow it to pass. Leland met earlier Tuesday with U.S. Customs officials in Laredo and spoke by phone with Treasury Secretary James Baker trying to persuade him to ease the ban on donating vehicles to Nicaragua, said convoy member Tom Hansen. About 30 tons of food, medicine and clothing are loaded on 38 vehicles. The Treasury Department's Office of Foreign Assets Control has told the convoy the cargo may be taken to Nicaragua, but that leaving the vehicles there would violate a trade embargo in effect with that leftist-ruled country for three years. Violating the embargo could result in fines of up to $50,000 and up to 10 years in prison. In addition, Condon said, Customs officials have told the convoy members that if they plan to go to Nicaragua without intending to return the vehicles within 30 days, the vehicles would be seized and the drivers subject to arrest. Condon said Tuesday the convoy did not intend to violate any laws, but added that the group was prepared to face a seizure when it reached Customs at Laredo's Bridge No. 1 today. Convoy members contend that the vehicles, mostly pickup trucks, are a form of humanitarian aid exempted from the Nicaraguan Trade Control Regulations. Asner spoke on the phone to convoy participants Tuesday. The nighttime rally also included the reading of a message Jackson sent to Baker, urging him to allow the convoy to pass. ``I hope that you will be guided by their example, expressed in their words, `Feed the children, not the war,''' the Mailgram from Jackson said. The convoy left from four different regions of the United States on May 21 and has been camped in Laredo since June 7. The U.S. government, which supports the Nicaraguan Contras fighting the leftist Sandinista government, imposed the trade control regulata in May 1985.