Barry Porter, Ujung Pandang – Armed supporters of the Golkar party ran amok in South Sulawesi yesterday, stoning opposition party offices and hurling insults about their leaders.

"Kill Amien Rais. Kill Amien Rais," yelled one small gang carrying machetes outside the National Mandate Party's (PAN) office in Ujung Pandang, as they lobbed missiles while police lamely stood by.

Security forces moved in to break up a similar disturbance outside the Indonesian Democratic Party of Struggle office, where Golkar supporters changed their chant to "Megawati is a whore". The city's party chairman had to restrain Megawati Sukarnoputri's followers from reacting.

In a separate incident, a supporter of the Indonesia Uni Democratic Party was attacked and left with a serious head injury.

The ugly scenes followed what had been a peaceful 1,000-strong rally for Golkar, which has governed the country for more than 30 years and for which Sulawesi is a key heartland. Golkar chairman Akbar Tanjung gave a speech in which he appealed for order.

One of Indonesia's independent election watchdogs has been investigating reports Golkar supporters carried guns during a weekend rally in Ujung Pandang.

The party has already twice been warned it may be barred from further campaigning if its supporters continue to bear arms.

Golkar has been the subject of more than half the complaints received by election watchdogs. As well as complaints about its arming of campaign officials – with machetes and spears as well as guns – party workers have been accused of bribery.

Independent political analyst Wimar Witoelar said: said: "This is very dangerous. If Golkar wins at the polls the public will remember [it cheated], which will be inflammatory."

There have been many cases of voter registration fraud, with some company bosses registering their staff in order to get multiple votes at their employees' expense.

In Sumenep and Mojokerto in East Java, thousands of ballot papers were found already punched. Teachers assigned to sort ballot papers were suspected of taking them home to mark. And in North Sumatra, 11 plastic bags containing 44,000 ballot papers were found drifting down a river.

Panwaslu, the general election supervisory committee, served Golkar's Jakarta branch with an official warning letter on Wednesday threatening to ban it from campaigning if it did not disarm its civilian security team.

The committee has also given Golkar and most of the other 48 parties contesting the June 7 vote written warnings for staging countless illegal street rallies.

So far the campaigning has been conducted largely in good spirits, often in a carnival-like atmosphere.