Troops opened fire on protesters in La Plaza de las Tres Culturas

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More than 25 people have been killed during a vicious gun battle in Mexico City just days before the Olympic Games are due to begin.

Thousands of students had gathered for a meeting organised by the National Strike Council in La Plaza de las Tres Culturas in Tlatelolco to protest against the military occupation of the National Polytechnic Institute.

The protesters, many of whom were women and children, had been planning to march through a working-class suburb of the city, but by early evening military personnel in armoured vehicles had surrounded the square.

The Mexican government say "agitatorgroups" among the students began shooting at the crowds from buildings, which resulted in a 90-minute gun fight.

General Marcelino Garcia Barragn, Mexico's defence minister said the army began firing into the crowd in self-defence after they found themselves targets of sniper fire from buildings in the square.

But several eye-witnesses claim the army entered the square in seven or eight armoured tanks and began shooting first.

After the fighting had subsided dozens of bodies lay strewn across the square, many more were injured.

More than 500 people have been arrested.

The violence follows weeks of demonstrations by students demanding democratic reform and social justice. They have used the international focus on Mexico City because of the Olympics to promote their message.

In September, President Gustavo Ordaz, in a bid to suppress the protests and cause minimum disruption to the Olympics, ordered the military occupation of the National Polytechnic Institute in Mexico City.

At this stage it is not clear whether the 7,000 athletes, currently preparing for the Games 11 miles away from Tlatelolco in the Olympic Village, are in danger. It is the first time the Olympics have been held in a Latin American country.

Lord Exeter, British vice-president of the International Olympic Committee told the Times: "The riots have nothing to do with the Olympic Games. The students are not protesting against the games but against the Mexican government." 

Monkeys have shown 'disrupted brain wave patterns'

The US government has funded the cultivation of marijuana in the southern United States for research purposes.

The drug was prepared and dried before being sent on to more than 50 medical schools and research centres to study its effects on animals and humans.

Researchers in New Orleans claim the active ingredient of cannabis, THC, has had severedetrimentaleffects on laboratory animals.

But supporters say cannabis is no more harmful or less harmful than drinking alcohol or smoking tobacco.

In a laboratory in New Orleans, rhesus monkeys were forced to smoke the equivalent of two cannabis cigarettes a day for nearly a year.

Electrodes implanted in their brains measured brain activity, and the results have indicated major if not permanent damage.

Researchers claim cannabis has a lasting impact - one monkey had not been subjected to the testing for six months, but still showed 'disrupted brain wave patterns'.

Studies on humans have assessed the effects of cannabis smoking on hormonal disruption, birth defects and severe personality disorders.

BBC correspondent John Humphreys spoke to two Vietnam War veterans taking part in the study who were heavy cannabis smokers of cannabis while on active duty.

"I just can't get my thoughts together quick enough. When you're stoned a problem can come up and it takes a while to get your stuff together to be able to deal with it. I'm still the same way now," said one.

Cannabis or marijuana is commonly used in the US - an estimated one in every eight Americans - 25 million people - have smoked the drug, and two million teenagers are regular users.

The drug is illegal in the United States for possession and consumption, and conviction for an offence often leads to a prison sentence.

agitator: one who agitates; a political troublemaker（煽动者）

detrimental :causing harm or injury（有害的）