Supporters of Lungu's Patriotic Front (PF) attacked an open-top campaign bus of Hichilema's United Party for National Development (UPND
Election officials and candidates in Zambia's presidential vote on Tuesday condemned fresh outbreaks of violence between rival supporters, as concerns grow that the relatively stable country could face worsening unrest.
Just 18 months after President Edgar Lungu narrowly won a snap election, he and the main challenger Hakainde Hichilema face off again in Thursday's poll in a field of nine candidates.
The stakes are high, with Lungu battling to retain the office he secured only last year, and Hichilema pushing to finally secure victory after four previous attempts.
On Monday, supporters of Lungu's Patriotic Front (PF) attacked an open-top campaign bus of Hichilema's United Party for National Development (UPND) in the Mtendere district of Lusaka.
UPND activists fled as rocks smashed into the vehicle, internet video footage showed, while several injuries were reported during other skirmishes in the district.
"The acts of violence that have characterised the 2016 elections campaigns are unprecedented and have marred Zambia's historic record of peaceful elections," election commission chief Esau Chulu said in a statement.
Last month, the commission halted campaigning in Lusaka for 10 days in an effort to reduce the regular clashes.
- 'Unacceptable' -
"Zambia is a peaceful nation," Lungu said Tuesday. "I will not tolerate any person attempting to break the peace we have.
"What happened in Mtendere yesterday is unacceptable. I have called on the police to... enforce laws on any person who will be found guilty irrespective of their political affiliation."
Only 27,757 votes -- less than two percentage points -- separated Lungu and Hichilema in the 2015 ballot.
With new constitutional rules demanding that the winner needs more than 50 percent, a second round run-off could be held weeks after the election, heightening hostilities further.
Election-related violence has erupted often in recent months, with Hichilema's loyalists furious at alleged attempts by the government to discourage opposition campaigning.
The UPND's vice president has been arrested and released twice, and the main independent newspaper has been closed in an apparent dispute over taxes.
"The PF shamelessly obstructs our campaigns by intimidating media outlets, leaning on the police to cancel our rallies and sponsoring attacks on our supporters," Hichilema told AFP by email.
"We have seen unarmed supporters shot dead by police and our youths beaten to death for no other crime than wearing a UPND T-shirt."
- Economic woes -
Zambia, ruled by Kenneth Kaunda from 1964 until 1991, has suffered a sharp drop in economic growth and thousands of job losses in its crucial copper-mining sector.
GDP growth last year was 3.6 percent, down from 10 percent in 2010, while inflation is over 20 percent.
"Both sides say that only a rigged election could stop them winning," Neo Simutanyo, director of the Centre for Policy Research, told AFP.
"The level of tension is higher than in the past, and the environment is not conducive to peaceful and fair elections.
"We are worried our democratic record is threatened."
The election is being held after the 2015 vote gave Lungu the right to complete the term of the late president Michael Sata, who died in office of an undisclosed illness.
Felix Mwelwa, 53, a casual labourer in Mtendere, scene of Monday's unrest, said that tackling unemployment must be the priority for the next government.
"We are now a country of street traders, not a country of production," he told AFP, declining to reveal his choice of party.
"Youths have nothing else to do, so they join in with election fighting and the police don't stop it."
No polls have been published predicting the election winner.
Zambia also votes on Thursday to choose lawmakers and local councillors, and in a referendum on an amended bill of rights.