Key Inclusion Criteria:  LN Cohort  Clinical diagnosis of SLE by 2019 American College of Rheumatology and European League Against Rheumatism criteria. Diagnosis of 2018 Revised International Society of Nephrology/Renal Pathology Society classification (active focal or diffuse proliferative LN Class III or IV) confirmed by biopsy obtained ≤ 6 months prior to Screening or during Screening Period. Participants may co-exhibit Class V disease. Participants with de novo or relapsing disease may be eligible. Clinically active LN at Screening requiring/receiving immunosuppression induction treatment in the opinion of the Investigator. Proteinuria with UPCR ≥ 1 g/g based on one 24 hour urine collection during the Screening Period.  IgAN Cohort  Established diagnosis of primary IgAN based on kidney biopsy obtained any time prior to or during the Screening Period. Mean proteinuria ≥ 1 g/day on 2 complete and valid 24 hour urine collections during the Screening Period. Presence of hematuria as defined by 1+ blood based on urine dipstick or ≥ 10 red blood cells/high power field microscopy on urine sediment (performed by the local laboratory) during Screening Period (only applicable if diagnostic biopsy is >2 years prior to Screening). Compliance with stable and optimal dose of RAS inhibitor treatment including maximum allowed or tolerated angiotensin converting enzyme inhibitor and/or angiotensin receptor blocker dose for ≥ 3 months prior to Screening with no expected change in dose during the study (participants with established intolerance to RAS inhibitors may be included). Controlled and stable blood pressure (defined as < 140/90 millimeters of mercury [mmHg]) over the past 3 months prior to randomization.  Key Exclusion Criteria:  Both Cohorts  eGFR ≤ 30 milliliters/minute/1.73 squared meters during Screening calculated by Chronic Kidney Disease Epidemiology Collaboration. More than or equal to 50% interstitial fibrosis, tubular atrophy, glomerular sclerosis, or crescent formation in glomeruli on most recent kidney biopsy prior or during the Screening Period. Concomitant significant renal disease other than LN or IgAN on the most recent biopsy prior to or during the Screening Period. History of solid organ or bone marrow transplant, or planned transplant during the Extended Treatment Period (50 weeks). Splenectomy or functional asplenia. Known or suspected complement deficiency, unless attributable to underlying disease (that is, LN and IgAN). Bone marrow insufficiency with absolute neutrophil count < 1.3 × 10^3/microliter; thrombocytopenia (platelet count < 50,000/cubic millimeter).  For LN Cohort  Participants who have received any of the following treatments:  Cyclophosphamide ≤ 6 months prior to Screening Calcineurin inhibitors ≤ 3 months prior to Screening A cumulative dose of intravenous methylprednisolone > 3 g for the current active renal flare Mycophenolate mofetil > 2 g/day (or equivalent) for ≥ 4 consecutive weeks prior to Screening for the current active renal flare Prednisone or prednisone equivalent ≥ 0.5 milligrams (mg)/kilogram/day for ≥ 4 consecutive weeks prior to Screening for the current active renal flare Uncontrolled hypertension (systolic blood pressure > 160 mmHg or diastolic blood pressure > 110 mmHg) on 2 or more measurements during the Screening Period.  For IgAN Cohort  Diagnosis of rapid progressive glomerulonephritis as measured by eGFR loss ≥ 30% over a period of 3 months prior to or during the Screening Period. Secondary etiologies of IgAN. Prednisone or prednisone equivalent > 20 mg for > 14 consecutive days or any other immunosuppression within 6 months prior to Screening. Blood pressure of ≥ 140/90 mmHg during the Screening Period confirmed on 2 measures > 30 minutes apart.